Define %python3_cache_tag / %python_cache_tag, e.g. cpython-311
    
    When reviewing https://src.fedoraproject.org/rpms/pyproject-rpm-macros/pull-request/291
    we have discovered that there is no macronized way to get this part of some paths
    and that packagers need to hardcode it as cpython-%{python3_version_nodots}.
    
    This way, we have a standardized macro packagers (and other macros) can use.