Cherry-pick #472 to main: Find Python3 directly, not with GzPython

[scpeters]

PR #472 is a fix that will unbreak several CI jobs on macOS. It will be merged forward to main in #479, but that is slowed down by issues with commented tests, so I am cherry-picking this so expedite the fix of CI.

Commit message:

This removes the use of gz-cmake's GzPython in favor of a single call to find_package(Python3). This allows setting Python3_EXECUTABLE to specify which python version to use and is needed on macOS.

Note to maintainers: Remember to use Rebase-and-Merge.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (30560f5) 87.78% compared to head (3f14b54) 87.82%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #480 +/- ## ========================================== + Coverage 87.78% 87.82% +0.03% ========================================== Files 59 59 Lines 5699 5699 ========================================== + Hits 5003 5005 +2 + Misses 696 694 -2 ```

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