Make tests pass for people who aren't Colin

[smcv]

• git-evtag-compute-py: Port to Python 3

  Python 2 reached EOL in January 2020.

• tests: Configure git author/committer name

  Otherwise, recent versions of git will refuse to commit anything on an unconfigured system.

  Use Colin's name and email address here for now, rather than a dummy set of data. This is because the EVTag covers these, so if we don't use the same, we can't possibly get a match for the hard-coded expectation.

• tests: Make fewer assumptions about git configuration

  This test assumes git is configured with push.default=simple or push.default=matching, but some developers like to configure it with push.default=nothing to avoid mistakes.

• tests: Compare C and Python implementations before making assertions

  It's helpful to be able to see the results of each implementation, and whether they agree, before comparing them with the reference.

• tests: Output valid TAP

  The number of tests planned wasn't updated when more tests were added.

• tests: Run at build-time

  If we can catch bugs at build-time, then the package doesn't need to be included in a distribution so that its as-installed tests can run.