mail: suppress a pytype error that's just experimentally wrong

It's probably more correct to pass a str here, but I'm not going to
worry about it for now.

Differential Revision: https://phab.mercurial-scm.org/D7283

mercurial/mail.py

@@ -279,7 +279,9 b' def mimetextqp(body, subtype, charset):'
     '''Return MIME message.
     Quoted-printable transfer encoding will be used if necessary.
     '''
-    cs = email.charset.Charset(charset)
+    # Experimentally charset is okay as a bytes even if the type
+    # stubs disagree.
+    cs = email.charset.Charset(charset)  # pytype: disable=wrong-arg-types
     msg = email.message.Message()
     msg.set_type(pycompat.sysstr(b'text/' + subtype))