Configuring pytest to pick up "*.py" accidentally pulls in setup.py

As noted in NexB/saneyaml#15, the `pytest` configuration in `pyproject.toml` currently configures it to pick up `*.py`:

https://github.com/nexB/skeleton/blob/acf94b360d675087195b7f216f4bee4401609729/pyproject.toml#L10
https://github.com/nexB/skeleton/blob/acf94b360d675087195b7f216f4bee4401609729/pyproject.toml#L43

This causes it to pick up the `setup.py` file as a potential test file, and even though it will contain no tests, importing `setup.py` during testing can have unwanted side effects. (For one, it makes `setuptools` a dependency of the test environment, where it would otherwise normally not be.)

Unfortunately, reading through [the pytest documentation](https://docs.pytest.org/en/8.2.x/reference/reference.html#ini-options-ref), it doesn't seem as though there's an easy way to exclude specific files once they've been caught up in an inclusion glob.