remove unnecessary "__provides_exclude_from" and custom definition of "python3_pkgversion"
    
    "__provides_exclude_from" macro is unnecessary in Fedora and EPEL 7.
    See [1] for detailed explanation.
    
    All currently supported versions of Fedora provide "python3_pkgversion"
    out of the box (it exists since Fedora 24 or 25) so we can remove the
    extra line here.
    
    [1] https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/K3NLDLK4XVARTSROC6XVKNVW72BMMHYE/