gh-102362: Fix macOS version number in result of sysconfig.get_platform() (GH-112942)

Change _osx_support.get_platform_osx() to make sure that the
version number in the result includes at least a major and
minor version (e.g. 14.2) even if MACOSX_DEPLOYMENT_TARGET is
set to just a major version (e.g. 14).

This matches the versions expected by pip when selecting
appropriate wheels for installation.