Idea: provide default gh-actions config for simple projects

[ymyzk]

tox-gh-actions may be able to implicitly assume configurations for simple projects. e.g.,

[gh-actions]
python =
    2.7: py27
    3.6: py36
    3.7: py37
    3.8: py38
    3.9: py39

[hugovk]

Yes please, this would be great!

The supported Python versions are already listed for projects in quite a few places (e.g. GHA yml, Travis yml, setup.py classifiers, tox.ini, docs), it would make things even more seamless not to need to list it in yet another place, and make upgrades easier.

https://github.com/tox-dev/tox-travis does something similar.

[bittner]

This might allow to repurpose (or do away with) the fail_on_no_env = True setting.

1. If no python key is specified in [gh-actions] then assume the default (CPython-based) mapping, e.g. 3.10 = py310, etc.
2. If a python key is specified and not all cases are listed in the mapping then warn (or print out an info) for the missing cases; if fail_on_no_env = True fail for missing cases.