basic approach for separate pytest plugin package

[bollwyvl]

• fixes pytest_tornasync becomes hard dependency if pytest is installed #337

The simplest packaging I could do. Probably needs something to keep the parent/plugin versions in sync, and a release approach.

[kevin-bates]

Thank you Nick! Given we're packaging the fixtures in https://github.com/jupyter-server/pytest-jupyter, I believe some of these changes are probably warranted there (e.g., use of options.entry_points). It would be great if you could take a look at the packaging aspects on that repo.

Others of these commits have been addressed in #335.

We were going to remove the entry point in a follow-on PR, post #335, which is #339, so I'm thinking the spirit of this PR is really what has happened in the pytest-jupyter repo and we should apply any additional necessities there.

Are you good with closing this PR, while focusing on #335 and #339 here, with some additional focus (relative to packaging, etc.) in pytest-juptyer?

[bollwyvl]

Thanks for super quick review! Will investigate those links this weekend, hopefully!