py3: cast decode() argument to system string

The actual types here may be mixed due to string literals
and variables from other modules. So a cast is necessary.

Differential Revision: https://phab.mercurial-scm.org/D2166

mercurial/mail.py

@@ -206,7 +206,7 b" def mimetextpatch(s, subtype='plain', di"
         return mimetextqp(s, subtype, 'us-ascii')
     for charset in cs:
         try:
-            s.decode(charset)
+            s.decode(pycompat.sysstr(charset))
             return mimetextqp(s, subtype, codec2iana(charset))
         except UnicodeDecodeError:
             pass