Use python%{python3_pkgversion}-devel as BR in EPEL 7
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This merge request is automatically copied from !674 (merged) orginally made by @ellert
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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.