Avoid python dependency in git-core (#1536471)
    
    Move contrib/hooks/multimail from git-core to git and use python3 rather
    than python2.
    
    We still use python2 as the default PYTHON_PATH because not all of the
    python scripts in git support python3.  None of the other scripts are
    included in git-core though.
    
    Primarily, python2 is used by git-p4 and contrib/svn-fe/svnrdump_sim.py
    (which is used by t9020-remote-svn.sh).  Converting git-p4 to python3 is
    not a trivial matter of fixing a few print statements.  A simple
    conversion using python3's 2to3 tool results in a large number of test
    failures.
    
    Add a python3-devel BuildRequires for %{__python3} and add python2-devel
    to the tests section since t9020-remote-svn.sh uses python2.  (We
    already BR python2-devel in git-p4, but having it in the tests section
    ensures we don't remove it if/when git-p4 supports python3.)