python bindings: get version from package.xml

[scpeters]

🎉 New feature

Implements feature request from #636 (comment) using gazebosim/gz-cmake#456 from gz-cmake 4.1.0, replaces prototype in #639

Summary

When building python bindings separately from the main library, the major version of gz-math has been hard-coded, but this changes it to parse the gz-math version number from the package.xml file in this repository using a gz-cmake feature added in gazebosim/gz-cmake#456 and released in 4.1.0. We could also consider using this feature in the root CMakeLists.txt file, but for now it is just used when building python bindings separately from the main library.

Test it

1. Configure the src/python_pybind11 folder:

cd src/python_pybind11
mkdir build
cd build
cmake ..

2. Verify that it found gz-math8 and configured with version 8.0.0

-- Looking for gz-math8 -- found version 8.0.0

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

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