py3: use native strings when forming email headers in patchbomb...
py3: use native strings when forming email headers in patchbomb
Per previous changesets, encoded header's values are native str. We
complete the change in patchbomb extension to have literal header values
native str as well. Then we can also change headers' keys to be str. In
_msgid(), we still need to use encoding.strfromlocal() because usage of
os.environ is not allowed by check-code.
This finally removes the "if pycompat.ispy3:" TODO.