Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Steven Silvester <steven.silvester@ieee.org>

Closes #823

SHA256 hashes:

traitlets-5.8.1-py3-none-any.whl: a1ca5df6414f8b5760f7c5f256e326ee21b581742114545b462b35ffe3f04861

traitlets-5.8.1.tar.gz: 32500888f5ff7bbf3b9267ea31748fa657aaf34d56d85e60f91dda7dc7f5785b

* Polishing argcomplete support

* Forgot to add license notice on new files
* Add support for completing subcommands. Its not easy to
  do this with argparse's subparser with the way traitlets
  uses argparse, so instead directly add subcommands as
  completions for the first cword in argcomplete.
* A few other small refactor/cleanup/testing

* Add missed subapp.clear_instance() in unit test

Otherwise, this test prevents other tests from creating
subcommand applications.

Alternative to #825, that adds tests for behavior.

I don't think we _need_ the ability to set the value via a constructor,
And I'm generally in favor of simplifying, but at least this should have
tests.

SHA256 hashes:

traitlets-5.9.0-py3-none-any.whl: 9e6ec080259b9a5940c797d58b613b5e31441c2257b87c2e795c5228ae80d2d8

traitlets-5.9.0.tar.gz: f6cde21a9c68cf756af02035f72d5a723bf607e862e7be33ece505abf4a3bad9