Remove dependency on GnuPGInterface

[Barry Warsaw]

Currently duplicity uses GnuPGInterface to provide Python support for GPG. Upstream duplicity contains its own copy while in Ubuntu, this is patched to use the python-gnupginterface package.

The problem with this is that GnuPGInterface is (AFAICT) no longer being maintained, and thus will never support Python 3.

This will make it easier to port duplicity to Python 3 later.

One possibility is to use python-gnupg which is supported for Python 2 and 3, but it has a completely different API that won't make a port very easy. The alternative is just to wrap subprocess directly. I've tried the former and failed, so now I'm going to try the latter.

[Kenneth Loafman (kenneth-loafman)]

I patched GnuPGInterface to harvest the GnuPG tasks as soon as they were complete rather than wait for the iterators to complete and harvest them then. You will need to be aware of that patch otherwise folks with long incremental chains will run out of resources because GnuPG tasks have not been harvested in a timely manner.

[Kenneth Loafman (kenneth-loafman)]

We renamed the file a while back so we 'own' it, and we will make the needed changes when porting to Python 3.