Allow to pass environment variables to tox environments

[gforcada]

So:

[test]
pass_env =
    MY_CUSTOM_VARIABLE
    ANOTHER_ONE
    PYTHONWARNINGS

Specially the last one, when not wishing to remove deprecation warnings helps a lot in not drowning within them, when you are actually looking at why your tests failed 😅

And what's best: that we allow passing them, does not mean that we are enabling them, you should still do:

export PYTHONWARNINGS="ignore::DeprecationWarning"

✨