modernize conda_install test on Circle CI by installing from conda-forge with Python 3.9 and change install instructions in documentation

[valeriupredoi]

Description

Closes #1279
conda install works fine with these changes, hopefully it'll not run out of memory anymore, at least as often as before 🍺

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Codecov Report

Merging #1280 (c2ba2cb) into main (92371de) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1280   +/-   ##
=======================================
  Coverage   86.39%   86.39%           
=======================================
  Files         188      188           
  Lines        9167     9167           
=======================================
  Hits         7920     7920           
  Misses       1247     1247           

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[bouweandela]

Thanks for taking care of this! Could you also check that these steps are identical to the documentation?

[valeriupredoi]

Thanks for taking care of this! Could you also check that these steps are identical to the documentation?

no, good point, man - we need to remove the use of esmvalgroup channel for Core in the documentation, should I do that here or in a different PR (I'd do it in a separate PR right away if it was by me)

[bouweandela]

It would be great if you could do it here, the tests on CircleCI/GitHub Actions should always reflect the documentation. That way we are sure that the instructions in the documentation actually work.

[valeriupredoi]

right on @bouweandela - will do it 'ere then 👍

[valeriupredoi]

OK Bouwe, done that, could you have a looksee pls mate, I've also changed a bit the instructions on how to build&upload the package 🍺

[valeriupredoi]

cheers @bouweandela 🍺