Use python%{python3_pkgversion}-devel as BR in EPEL 7

This is the preferred way to state the BuildRequires on the default Python 3 version in EPEL 7. The update with the rebuild of all pythin 3 packages in EPEL 7 is now in the epel-testing repo. This way of stating the BR was used by those that were doing the mass rebuild of the Python 3 packages in EPEL 7 (incuding ARC). It actually works both before and after the change, so it is nit necessary to wait with the merge until the Python version change is in EPEL stable.