merge pytest plugin

[obestwalter]

This is basically an enhanced duplicate of emilybache/pytest-approvaltests#2 as the fork in this org is newer, but the issues are in the original repository.

Instead of maintaining the plugin in a different project and make it necessary to install it extra, it could be a part of the main repository. hypothesis is doing the same thing. The Python implementation of hypothesis always ships with the pytest plugin and therefore brings its pytest support out of the box.

I would propose to merge all open PRs from https://github.com/approvals/ApprovalTests.Python.PytestPlugin/pulls and https://github.com/emilybache/pytest-approvaltests/pulls and make sure the issues at https://github.com/emilybache/pytest-approvaltests/issues are handled.
then merge the fork into the main code, archive the original repo and delete the fork in this org.

advantages:

• more convenient for pytest users: they only have to install approvaltests (users who do not use pytest, have no disadvantage either)
• no problems with incompatible versions of main project and plugin
• less maintenance overhead - all we add to the main project (static code analysis, code formatting, etc.) applies to main code and plugin
• easier release management - one release for main and plugin

I can only think of very hypothetical disadvantages, which could be solved in the worst case by unbundling the plugin in a later release.