Update pin on esmvalcore pick up esmvalcore=2.3.0

[valeriupredoi]

Description

• Closes Update ESMValCore pin to new release #2199
• Works around this failure by pinning matplotlib to an older version and nc-time-axis restricted to not be 1.3.1 (even if that's currently impossible to be installed with iris as explained in this issue nc-time-axis==1.3.1 impossible to install with scitools-iris==3.0.2 SciTools/nc-time-axis#71 - the new nc-time-axis may be resolvable in the future with iris, but the plotting issue may still persist)
• add setup.py to flake-8-ignore list

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[zklaus]

Great, thanks! Just please do the checklist!

[valeriupredoi]

Great, thanks! Just please do the checklist!

shall do, boss! Just waiting on the tests 👍

[valeriupredoi]

conda build running out of resident memory again and the other tests taking an eternity, might be finished by the time Wales-Turkey game starts 🏴󠁧󠁢󠁷󠁬󠁳󠁿 - 🇹🇷 I'll try rerun the conda build test again, hopefully it was a fluke, but if it's persistent we'll just have to merge this and deal with it separately

[valeriupredoi]

looking into conda build consuming copious amounts of memory now

[valeriupredoi]

OK - tests pass, env gets created, etc etc (and I pinned Matplotlib due to conflicts with nc-time-axis, see above) - only thing that is a showstopper is (again) conda-build that's going through the roof on memory - I ran some local tests and can't throttle the memory just yet, @zklaus you OK to merge this now and I'll keep working on the conda-build memory sink? I think this is starting to be needed by others

[zklaus]

Yes, I am ok with that.

[valeriupredoi]

hang on I think I've fixed it

[zklaus]

If you want to experiment, that's fine, but I am not prepared to leave Python 3.8 behind just yet. In fact, I am not completely sure about Python 3.7, but that might be ok. All in all, if the problem really is only with the conda-build, I would highly recommend to merge this PR soon, preferably with Python>=3.7, and continue any necessary work on the conda-build separately.

[valeriupredoi]

I will right after the test (hopefully) passes, then I will revert back to 3.7 and merge, gimme 10min max, man

[zklaus]

Start the clock 👅

[valeriupredoi]

OK Ive reverted the test bits (looks like with Python 3.9 only conda build goes on w/o memory issues) - waiting for all tests bar conda build to finish and then immediately merge