Fix python macro memoizing to account for changing %__python3
    
    This adds a new `%_python_memoize` macro that caches the values of
    various python macros in a lua table based on the current value of
    `%__python` / `%__python3`. The Python macros are adjusted to use this
    macro for memoization instead of the current approach. This way,  the
    macros will return values that apply to the _current_ `%__python3` value
    when packagers create multi-python specfiles that toggle the value of
    `%python3_pkgversion`.
    
    Relates: https://bugzilla.redhat.com/2209055
    
    Co-Authored-By: Miro Hrončok <miro@hroncok.cz>