Feature: reuse PDM scripts

[noirbizarre]

This pull-request add support for reusing PDM scripts in tox for both versions 3 and 4.
The following have been achieved:

• enable tox testing on both versions 3 and 4
• implement PDM scripts detection on both tox 3 and 4 (and add integration tests for those use cases)
• move project tox commands into reusable PDM scripts and use them in tox
• update the README.md to advertise this feature

Note: for tox4, given there is not yet any recommandation, guidelines or official API... I chose one possible implementation but I had many implementation working. One was simply added into the _setup_env method but it was feeling hackish and sementically wrong

Note: given test_integration.py is already using tox to test, I'm wondering if it still makes sense to call tox from CI as pdm run test should be enough (will run on the Py version and the tox version from CI). I had to update the CI workflow as the tox4 jobs where not really using tox4. I added a tox --version to be able to check the version really in use

Last note: The coverage change in pyproject.toml is using this fix until next pdm version is released including pdm-project/pdm#1112