pyme does not support Python 3.  It looks like the latest version of
that library, renamed, which does support Python 3, is:

https://dev.gnupg.org/source/gpgme/browse/master/lang/python/
https://pypi.org/project/gpg/

So Roundup should switch to using that in place of pyme when available,
which requires updating the pyme references in doc/installation.txt.

Note: as a Python 3 issue for the mail interface, it probably makes
sense first to update and merge the patches mentioned in issue2550970 to
stop using mimetools, as that's needed for the mail interface to be
usable at all with Python 3, with or without gpg support.