Pin importlib_metadata <8

[ehogan]

When creating a new environment for testing ESMValCore v2.11.0rc2, the issue I reported at #3208 (comment) was present. @valeriupredoi identified that importlib_metadata was the culprit:

(rc1-env) % conda list importlib_metadata
# packages in environment at /miniforge3/envs/rc1-env:
#
# Name                    Version                   Build  Channel
importlib_metadata        7.1.0                hd8ed1ab_0    conda-forge
(rc2-env) % conda list importlib_metadata
# packages in environment at /miniforge3/envs/rc2-env:
#
# Name                    Version                   Build  Channel
importlib_metadata        8.0.0                hd8ed1ab_0    conda-forge

The solution is to add the importlib_metadata dependency to ESMValTool and pin the version to <8.

Note that importlib_metadata will be removed in ESMValGroup/ESMValCore#2447; the removal should also be added to #3683 after the release / before the release branch is merged into main 👍

[valeriupredoi]

just to add to this retroactively - this shouldn't be a problem anymore once esmf/esmpy go past 8.6.0 pin (currently blocked by NCL, and it may be a while until it gets unblocked) - adding info since I introduce the pin in main via #3501 - CC @bouweandela