typing: restore `encoding.encoding` and `encoding.encodingmode` to bytes

Somewhere between hg 3dbc7b1ecaba and hg 8e3f6b5bf720, pytype determined the
signature of these fields changed from `bytes` to `Any`.  Not sure why- the type
of `environ` then and now is: `Union[WindowsEnviron, Dict[bytes, bytes], os._Environ[bytes]]`
That said, PyCharm wasn't able to figure out the type of `environ`, and the
`WindowsEnviron` class extends `MutableMapping` without specifying bytes for the
key and value types in py3.9.  But that's not changed in my setup, so I can't
explain it.

mercurial/encoding.py

@@ -103,14 +103,15 else:
 if pycompat.iswindows:
     _encodingrewrites[b'cp65001'] = b'utf-8'
 
+encoding: bytes = b''  # help pytype avoid seeing None value
 try:
-    encoding = environ.get(b"HGENCODING")
+    encoding = environ.get(b"HGENCODING", b'')
     if not encoding:
         encoding = locale.getpreferredencoding().encode('ascii') or b'ascii'
         encoding = _encodingrewrites.get(encoding, encoding)
 except locale.Error:
     encoding = b'ascii'
-encodingmode = environ.get(b"HGENCODINGMODE", b"strict")
+encodingmode: bytes = environ.get(b"HGENCODINGMODE", b"strict")
 fallbackencoding = b'ISO-8859-1'