This sets up GitHub Actions (GHA) to run in place of the
currently broken Travis CI. Initially, this only covers running
tox/pytest and Black, but may eventually be extended to run pylint,
mypy, flake8, etc. - see #605, for example.
Notes:
* Python 3.10 is not yet supported due to the `collections.Iterable`
issue discussed in #330, #624, etc.
* The Black CI step acts as a linting step, rather than attempting to
have the GHA job automatically update/commit/push the reformarted
code.
* Black is currently pinned to an older version that supports
`--target-version py27` until Python 2 compatibility can be dropped in
the final Python 2 compatibility release of ffmpeg-python.
* Only the main source directory (`ffmpeg/`) is checked with Black at
the moment. The `examples/` directory should also be checked, but
will be done as a separate PR.
