[frontend] Byecompile files in %{_datadir} with python3
    
    See https://fedoraproject.org/wiki/Packaging:Python_Appendix#Manual_byte_compilation
    
    Currently, it was done with the default /usr/bin/python with this result:
    
    $ file /usr/share/copr/coprs_frontend/tests/test_views/test_coprs_ns/test_coprs_builds.pyc
    /usr/share/copr/coprs_frontend/tests/test_views/test_coprs_ns/test_coprs_builds.pyc: python 2.7 byte-compiled
    $ file /usr/share/copr/coprs_frontend/tests/test_views/test_coprs_ns/test_coprs_builds.pyo
    /usr/share/copr/coprs_frontend/tests/test_views/test_coprs_ns/test_coprs_builds.pyo: python 2.7 byte-compiled
    ...
    
    This also adds BR python3-devel to:
    
     * ensure /usr/bin/python3 is present even if tests are disabled,
     * follow the guidelines.