Change EPEL 7 python 3 version to 3.6

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This merge request is automatically copied from !651 (merged) orginally made by @ellert
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EPEL 7 has changed which python 3 version is the default (i.e. the one that is referenced by the %__python3 macro in the rpm config). This adapts the specfile to this change.

Should be ported to ARC5 if ARC5 nightlies for EPEL 7 should succeed.

(Failed in koschei - fedoras CI, but not yet in our nighlies because the change has not propagated to the EPEL repo mirrors yet.)

added port:ARC5 xarcbot-copied:next + 1 deleted label

merged

mentioned in commit 27364dae

removed port:ARC5 label

added component:building component:packaging type:build-fix labels and removed 1 deleted label