CI: Re-add python as dependency
CI: Re-add python as dependency

clang-tidy needs Python 2.7 and argparse, but SLC6 only has Python 2.6.
This made the SLC6 linter CI job silently fail (returning success)

Koensw/allpix-squared,Koensw/allpix-squared,Koensw/allpix-squared,Koensw/allpix-squared