Find python directly instead of using GzPython

[scpeters]

🎉 New feature

Part of gazebosim/gz-cmake#350.

Summary

I suggested in https://github.com/gazebosim/gz-cmake/issues/350#issuecomment-1907453341 that we find python directly so that GzPython can be deprecated, and this is a step towards that goal.

Test it

Compile and run tests and verify that the check_test_ran.py script still runs.

Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.