(v2) Extend tox.ini with testpaths and {posargs} in poetry-pytest invocation

[chrisinmtown]

This allows limiting the scope of the test run, which can save much time.

Run one test in one test environment:

tox -e py39 -- tests/test_api.py::test_api

Run one test in all test environments:

tox -- tests/test_api.py::test_api

Run all tests in one file in one test environment:

tox -e py39 -- tests/test_api.py

Run all tests in one file in all test environments:

tox -- tests/test_api.py