Refresh CI: add conda/mamba environment file and modern env creation, fix tests, and add modern Github Action workflow file

[bouweandela]

• Update tests to use modern Python versions
• Enable nightly test runs

[bouweandela]

It looks like the tests heavily rely on the availability of the CEDA ESGF index node, so let's wait until that comes back online.

[valeriupredoi]

do you want me to take over this PR, bud? If you don't have time, that is. It's well needed 😁

[valeriupredoi]

OK fairly serious upgrades here:

• a working conda/mamba environment that makes requirements and requirements_dev obsolete
• a decent testing environment built from conda-forge;
• fixes to a few tests;
• removal of old way of getting pandoc; notebook and docs build well for all Python versions

CAVEAT tests/test_results.py is pretty broken (I didn't break it, it was already when I took over this), so I turned it off for now, so that we have a modicum of tests run on PRs and nightly; those need fixed, but I don't have the expertise ro do it, so I'll open a PR 👍