restore, and from now on CI-test for entry point

[pajod]

1. We want the built package to install a "gunicorn" script.
   • Tell setuptools, now it reads that from pyproject.toml
   • Fixes: pyproject.toml does not declare entryopints. #3119
2. We want that script to work and reasonable output when called gunicorn --help
   • Have CI assert that
3. We want the entry point to work more or less interchangeably with python3 -m gunicorn (that is: __main__.py)
   • Have CI assert that

Driveby fixes:

4. GitHub actions finish faster when the setup-python knows which dependencies are reused
   • potentially unwanted effects for private dependencies - OK, we only use PyPi hosted (and occasionally public GitHub)
   • Verification: In section "Install Dependencies" logs will say "Using cached" in place of "Downloading"
5. Some python tools fail to agree with GitHub runners about console capabilities.
   • Enforce color output
   • Verification: I have seen it work.
6. Removed cryptography dependency again.
   • commit 91cb3dc does not justify including it, if it was indeed needed maybe it should be referenced elsewhere (somewhere not parsed by CI)
   • Verification: Removing it did not noticeably break or slow down CI
7. GitHub actions are versioned. If they improve things, we want that.
   • Since we do not depend on versioned features, we can just bump the major version and automatically use a recent release.
   • Verification: Did not noticeably break or slow down CI