drop dependency on mock >= 4.0

[richvdh]

We have promised our downstream packagers we will drop our dependency on mock >= 4.0 in Synapse 1.50, since at the time, we believed it to be redundant as of Python 3.7.

That doesn't actually seem to be the case (unittest.mock.__aenter__ and friends were only added in Python 3.8), but we still need to solve this problem (In short, Fedora doesn't package recent versions of mock: https://fedoraproject.org/wiki/Changes/DeprecatePythonMock.)

(for links: the dep on mock 4.0 was introduced in #11306).

[richvdh]

#11588 contains a fix for this.