Tests and documentation build are failing

[sloosvel]

Tests are failing because the environment is taking too long to solve (@valeriupredoi is it because of #3947 ?) and the documentation build is also failing. I guess for the second issue, sphinx also needs to be pinned as in ESMValGroup/ESMValCore#2671 ?

[valeriupredoi]

@sloosvel the tests are failing indeed because of a too long env solving but only in the tests with upstream development packages test since we desperately need mamba>=2 there too - I missed that one yesterday in the PR that fixed the other tests. I suspect the docs fail because of the same issue, I'll have a look 🍺

[valeriupredoi]

need to jump in a meeting in 10min but will open a PR fix these asap