The version of versioneer has an issue that the tarball that is generated on
github changes, when the branches and/or tags refering to a commit change. This
causes issues for gridsync, which pins the hashes of those tarballs.

In addition to that, versioneer doesn't provide a meaningful revision for
untagged commits, when run from a github tarball.

Given that we only ever consume a github tarball as a github archive (except in
local development, where the version number is less important), this replaces
versioneer with some hand-rolled code specialized to that use case.

This includes a minimal in-memory implemenation which can be used by tests, and
adds the appropriate calls to the new interface to the storage server
implemenation.

avoids ambiguities and implementation problems of iso8601 duration strings

go back to aniso8601 dependency, since we no longer get anything distinctive
from isodate

tahoe-lafs 1.15 depends on attrs<20
something else pulls in attrs 21.2.0
pip fails to resolve the dependencies correctly in this case

So optimistic...

Cannot figure out how to have a VCS or other non-package-name dependency w/ install_requires.

We can put a plugin here later.