Implemented cwd parameter

Revert "Implemented cwd parameter"

…-master

Revert "Revert "Implemented cwd parameter""

…ning#639)

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 kkroening#605, for example.

Notes:
* Python 3.10 is not yet supported due to the `collections.Iterable`
  issue discussed in kkroening#330, kkroening#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.

Co-authored by: Christian Clauss <[email protected]>

Co-authored-by: digitalcircuits <[email protected]>
Co-authored-by: digitalcircuits <digitalcircuits@github>

…ing#330)

* Import ABC from collections.abc instead of collections for Python 3.9 compatibility.

* Fix deprecation warnings due to invalid escape sequences.

* Support Python 3.10

Co-authored-by: Karl Kroening <[email protected]>

Co-authored-by: Karl Kroening <[email protected]>

(At least until Python 2.7 support is finally eliminated)