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.

seems to be a straightforward rename to use the new module:

https://bitbucket.org/cmeerw/roundup/commits/519572a328c4b6064f20985e1ed6
b692d983ba70

Note that commit is missing an update to doc/installation.txt where it 
mentions pyme.

merged into default: b7fa56ced601

(also updated references in docs now)

That doesn't seem to work that well with the ancient versions on the CI 
server... Question is, do/should we care?

I suppose it will be necessary for .travis.yml to build libgpgme11-dev 
locally (or use a suitable PPA if one exists, but I haven't found one).  
I think we certainly do want the CI to be able to run all the tests 
(without waiting for https://github.com/travis-ci/travis-ci/issues/5821 / 
https://github.com/travis-ci/travis-ci/issues/9460 to be fixed so we can 
just use a newer Ubuntu version in CI).

I think doc/upgrading.txt should also mention the new package you need to 
install if you were previously using the encryption support with pyme.

With the latest series of .travis.yml changes (up to a801eb13ab94), while 
the install of gpg doesn't fail, the relevant tests are still being 
skipped for some reason, so something about that package must be failing 
to work.

probably just the PATH or LD_LIBRARY_PATH, I have pushed a change to set 
those.

BTW, I have now also experimented with running the tests on circleci.com, 
see https://circleci.com/bb/cmeerw/roundup-tracker (they offer Debian 
stretch based images)

Thanks, that seems to have fixed it so again there are no tests skipped
in CI with Python 2.  I think we now just need an entry in upgrading.txt
(to tell people using this functionality that they need to install the
new gpg module to continue to use it with the next release of Roundup)
to consider this issue done.

added note to upgrading.txt

Thanks, so this is now fully done.