mail: let all charset values be native strings

Charset values will typically be used to build email.header.Header
instances, which takes str (though it tolerates bytes) or passed to
decode()/encode() methods of string values (which want str). It seems
that using native str involves less conversions than before and this
also helps type hinting (as illustrates removal of pytype disabling
instructions).

mercurial/mail.py

@@ -253,17 +253,13 b' def validateconfig(ui):'
 
 
 def codec2iana(cs):
-    # type: (bytes) -> bytes
+    # type: (str) -> str
     ''''''
-    cs = pycompat.sysbytes(
-        email.charset.Charset(
-            cs  # pytype: disable=wrong-arg-types
-        ).input_charset.lower()
-    )
+    cs = email.charset.Charset(cs).input_charset.lower()
 
     # "latin1" normalizes to "iso8859-1", standard calls for "iso-8859-1"
-    if cs.startswith(b"iso") and not cs.startswith(b"iso-"):
-        return b"iso-" + cs[3:]
+    if cs.startswith("iso") and not cs.startswith("iso-"):
+        return "iso-" + cs[3:]
     return cs
 
 
@@ -275,27 +271,30 b" def mimetextpatch(s, subtype=b'plain', d"
     ISO-8859-1, an encoding with that allows all byte sequences.
     Transfer encodings will be used if necessary.'''
 
-    cs = [b'us-ascii', b'utf-8', encoding.encoding, encoding.fallbackencoding]
+    cs = [
+        'us-ascii',
+        'utf-8',
+        pycompat.sysstr(encoding.encoding),
+        pycompat.sysstr(encoding.fallbackencoding),
+    ]
     if display:
-        cs = [b'us-ascii']
+        cs = ['us-ascii']
     for charset in cs:
         try:
-            s.decode(pycompat.sysstr(charset))
+            s.decode(charset)
             return mimetextqp(s, subtype, codec2iana(charset))
         except UnicodeDecodeError:
             pass
 
-    return mimetextqp(s, subtype, b"iso-8859-1")
+    return mimetextqp(s, subtype, "iso-8859-1")
 
 
 def mimetextqp(body, subtype, charset):
-    # type: (bytes, bytes, bytes) -> email.message.Message
+    # type: (bytes, bytes, str) -> email.message.Message
     '''Return MIME message.
     Quoted-printable transfer encoding will be used if necessary.
     '''
-    # Experimentally charset is okay as a bytes even if the type
-    # stubs disagree.
-    cs = email.charset.Charset(charset)  # pytype: disable=wrong-arg-types
+    cs = email.charset.Charset(charset)
     msg = email.message.Message()
     msg.set_type(pycompat.sysstr(b'text/' + subtype))
 
@@ -317,24 +316,25 b' def mimetextqp(body, subtype, charset):'
 
 
 def _charsets(ui):
-    # type: (Any) -> List[bytes]
+    # type: (Any) -> List[str]
     '''Obtains charsets to send mail parts not containing patches.'''
     charsets = [
-        cs.lower() for cs in ui.configlist(b'email', b'charsets')
-    ]  # type: List[bytes]
+        pycompat.sysstr(cs.lower())
+        for cs in ui.configlist(b'email', b'charsets')
+    ]
     fallbacks = [
-        encoding.fallbackencoding.lower(),
-        encoding.encoding.lower(),
-        b'utf-8',
-    ]  # type: List[bytes]
+        pycompat.sysstr(encoding.fallbackencoding.lower()),
+        pycompat.sysstr(encoding.encoding.lower()),
+        'utf-8',
+    ]
     for cs in fallbacks:  # find unique charsets while keeping order
         if cs not in charsets:
             charsets.append(cs)
-    return [cs for cs in charsets if not cs.endswith(b'ascii')]
+    return [cs for cs in charsets if not cs.endswith('ascii')]
 
 
 def _encode(ui, s, charsets):
-    # type: (Any, bytes, List[bytes]) -> Tuple[bytes, bytes]
+    # type: (Any, bytes, List[str]) -> Tuple[bytes, str]
     '''Returns (converted) string, charset tuple.
     Finds out best charset by cycling through sendcharsets in descending
     order. Tries both encoding and fallbackencoding for input. Only as
@@ -347,14 +347,17 b' def _encode(ui, s, charsets):'
         # wants, and fall back to garbage-in-ascii.
         for ocs in sendcharsets:
             try:
-                return s.encode(pycompat.sysstr(ocs)), ocs
+                return s.encode(ocs), ocs
             except UnicodeEncodeError:
                 pass
             except LookupError:
-                ui.warn(_(b'ignoring invalid sendcharset: %s\n') % ocs)
+                ui.warn(
+                    _(b'ignoring invalid sendcharset: %s\n')
+                    % pycompat.sysbytes(ocs)
+                )
         else:
             # Everything failed, ascii-armor what we've got and send it.
-            return s.encode('ascii', 'backslashreplace'), b'us-ascii'
+            return s.encode('ascii', 'backslashreplace'), 'us-ascii'
     # We have a bytes of unknown encoding. We'll try and guess a valid
     # encoding, falling back to pretending we had ascii even though we
     # know that's wrong.
@@ -369,29 +372,30 b' def _encode(ui, s, charsets):'
                 continue
             for ocs in sendcharsets:
                 try:
-                    return u.encode(pycompat.sysstr(ocs)), ocs
+                    return u.encode(ocs), ocs
                 except UnicodeEncodeError:
                     pass
                 except LookupError:
-                    ui.warn(_(b'ignoring invalid sendcharset: %s\n') % ocs)
+                    ui.warn(
+                        _(b'ignoring invalid sendcharset: %s\n')
+                        % pycompat.sysbytes(ocs)
+                    )
     # if ascii, or all conversion attempts fail, send (broken) ascii
-    return s, b'us-ascii'
+    return s, 'us-ascii'
 
 
 def headencode(ui, s, charsets=None, display=False):
-    # type: (Any, Union[bytes, str], List[bytes], bool) -> str
+    # type: (Any, Union[bytes, str], List[str], bool) -> str
     '''Returns RFC-2047 compliant header from given string.'''
     if not display:
         # split into words?
         s, cs = _encode(ui, s, charsets)
-        return email.header.Header(
-            s, cs  # pytype: disable=wrong-arg-types
-        ).encode()
+        return email.header.Header(s, cs).encode()
     return encoding.strfromlocal(s)
 
 
 def _addressencode(ui, name, addr, charsets=None):
-    # type: (Any, str, bytes, List[bytes]) -> str
+    # type: (Any, str, bytes, List[str]) -> str
     assert isinstance(addr, bytes)
     name = headencode(ui, name, charsets)
     try:
@@ -411,7 +415,7 b' def _addressencode(ui, name, addr, chars'
 
 
 def addressencode(ui, address, charsets=None, display=False):
-    # type: (Any, bytes, List[bytes], bool) -> str
+    # type: (Any, bytes, List[str], bool) -> str
     '''Turns address into RFC-2047 compliant header.'''
     if display or not address:
         return encoding.strfromlocal(address or b'')
@@ -420,7 +424,7 b' def addressencode(ui, address, charsets='
 
 
 def addrlistencode(ui, addrs, charsets=None, display=False):
-    # type: (Any, List[bytes], List[bytes], bool) -> List[str]
+    # type: (Any, List[bytes], List[str], bool) -> List[str]
     '''Turns a list of addresses into a list of RFC-2047 compliant headers.
     A single element of input list may contain multiple addresses, but output
     always has one address per item'''
@@ -440,10 +444,10 b' def addrlistencode(ui, addrs, charsets=N'
 
 
 def mimeencode(ui, s, charsets=None, display=False):
-    # type: (Any, bytes, List[bytes], bool) -> email.message.Message
+    # type: (Any, bytes, List[str], bool) -> email.message.Message
     '''creates mime text object, encodes it if needed, and sets
     charset and transfer-encoding accordingly.'''
-    cs = b'us-ascii'
+    cs = 'us-ascii'
     if not display:
         s, cs = _encode(ui, s, charsets)
     return mimetextqp(s, b'plain', cs)