Stage the Python license file during builds

[steadytao]

Summary

Stage the repo-root LICENSE file into the Python package directory during builds.

Problem

The Python package can no longer use ../LICENSE in license_files but the current license_files = LICENSE configuration also warns because python/LICENSE does not exist at build time.

Fix

• copy the repo-root LICENSE into the Python package directory before calling setup()
• let setuptools include it through license_files = LICENSE
• remove the staged copy afterwards so no tracked duplicate is needed in the repo

Testing

• ran python -m build in python/
• verified the previous license_files warning is gone
• verified the built wheel includes dist-info/licenses/LICENSE
• verified the temporary python/LICENSE file is removed after the build

Fixes #8789.