Restrict mock to <4

[valeriupredoi]

#453 mock=4 beta seems dodgy at the moment and the CI is picking it up from God knows where (nowhere on the conda forge or pypi official repos)

[valeriupredoi]

OK so it looks like we have to put up with the three test fails once we unpin mock -> they have actually released 4.0 testing-cabal/mock#476 but the updated packages have yet to arrive on pypi and conda-forge -> @zklaus can you pls have a look at #453 and advise as to how to proceed? This PR will do for in the meantime so we dont have our tests failed

[bouweandela]

This problem is already solved in #452.

[valeriupredoi]

as Bouwe says -> goto #452