tox -e docs: linkcheck very flaky

[obestwalter]

This check is definitely useful and should be done regularly, but sadly is very flaky.

This is what I can think of:

• remove it from normal builds and make this check a part of the release, still having a regular check for dead links but having less flaky builds.
• wrap this step to be a proper test and introducing pytest-rerunfailures to the mix.
• run these checks every time but not fail, if dead links are detected (my least favorite appproach, because those tests tend to be ignored then)

[gaborbernat]

what about 3 and 1 together 😄

[gaborbernat]

With the implementation of sphinx-doc/sphinx#7388 I think this should be less of an issue, so I'll enable the check unconditionally on the rewrite branch and see where it goes.