Switch to noxfile.py or toxfile.py

[fayazkhan]

Problem

The current configuration of SQLA versions to be tested is statically defined in tox.ini.
This doesn't cause any major code duplication or technical debt as the only other place this is constrained is in the requirements.txt.

However, this prevents @dependabot from functioning properly as it is not aware of the definitions in tox.ini. See #94.

Solution

To enable smooth functioning of @dependabot and also reduce code duplication, we should switch to a dynamic method of generating test environments.

nox seems to support this workflow. There also seems to be support added to newer versions of tox to achieve this using a toxfile.py similar to a noxfile.py from nox.

[fayazkhan]

Currently nox[uv] seems to be unable to work with pypy3.7:

nox > Running session test-pypy-3.7(sqlalchemy=0)
error: No download found for request: pypy-3.7-linux-x86_64-gnu
nox > Missing interpreters will error by default on CI systems.
nox > Session test-pypy-3.7(sqlalchemy=0) skipped: Python interpreter pypy3.7 not found.

So, maybe this switch would be less harmful when we drop support for it.