Investigate pins for various CI components (pytest-cov, coverage, msmb_theme) #3372
Description
Follow up from #3369
Related to #3224
Our docs build infrastructure is starting to be a bit flaky, particularly in #3369 we've had to pin pytest-cov due to some dependency resolution issues we were having when installing it at the same time as the sphinx components.
We should aim to check a) if all the pins are still necessary, b) what steps we should take to remove these pins.
My understanding is that some of this is related to the fact that we can't upgrade to the latest sphinx (see #3224), so we probably will want to fix that first.