Tox-gh-actions

* Fix warnings when packaging tox-gh-actions with [setuptools v54.1.0 or later](https://setuptools.readthedocs.io/en/latest/history.html#v54-1-0) 61

- Show tox env description in logs 55
- Add codecov to track test coverage 29
- Add basic integration test 54
- Improve CI for tox-gh-actions itself 52 53
- Improve examples in README 56

* Minor versions of PyPy are supported in the `python` configuration. For example, keys like `pypy-2.7` and `pypy-3.7` are supported in addition to `pypy2` and `pypy3`. 49, 50
* The `python` configuration supports versions only with a major part. For example, keys like `2` and `3` are supported. 51
* `testing` extras_require doesn't try to install black on PyPy 51

* Improve logging and explain how to investigate in README 46
* Test tox-gh-actions on more various environments 41
* Create tox-gh-actions' packages using Python 3.9 42
* Introduce black 43

- Add Python 3.9 support 33, 38
- Improve README

* Breaking change: When a list of environments to run is specified explicitly, tox-gh-actions now respects it and simply run the given environments without enforcing tox-gh-actions' configuration. 37
* Breaking change: Use `GITHUB_ACTIONS` environment variable instead of `GITHUB_ACTION` for enabling tox-gh-actions. 35
* Support GitHub Actions log lines grouping 36
* Add pytest-cov 28
* Improve documentation