Add tox posargs

[cdce8p]

Substitution with {posargs} allows devs to pass additional arguments to the tox command. Especially helpful if you like to run only a subset of tests for debugging. E.g

tox -e py313 -- --no-cov flit_core/tests_core/test_config.py

# would run
py313: commands[0]> python -m pytest --cov=flit --cov=flit_core/flit_core --no-cov flit_core/tests_core/test_config.py

https://tox.wiki/en/latest/config.html#substitutions-for-positional-arguments-in-commands