py3: decode encoding literal before passing to .decode()

bytes.decode(<encoding>) wants an str as "encoding" parameter,
it raises a TypeError if given a bytestring. encoding.encoding and
encoding.fallbackencoding are bytes values.

mercurial/mail.py

@@ -342,6 +342,7 b' def _encode(ui, s, charsets):'
         s.decode('ascii')
     except UnicodeDecodeError:
         for ics in (encoding.encoding, encoding.fallbackencoding):
+            ics = pycompat.sysstr(ics)
             try:
                 u = s.decode(ics)
             except UnicodeDecodeError: