Find python directly instead of using GzPython

[scpeters]

🎉 New feature

Part of gazebosim/gz-cmake#350.

Summary

I suggested in gazebosim/gz-cmake#350 (comment) 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.
• Compile and run python tests and verify that they pass.

Checklist

• 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.

[j-rivero]

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Otherwise, code changes looks good to me.