Fix PyPI publishing in CI

[ml-evs]

The new pip dependency resolver now actually resolves dependencies, causing mayhem. Somehow one of aiida's dependencies doesn't like the latest setuptools version, so this PR switches back to the legacy resolver temporarily.

Aims to close #624.

[codecov]

Codecov Report

Merging #623 (0417216) into master (f79da65) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #623   +/-   ##
=======================================
  Coverage   92.20%   92.20%           
=======================================
  Files          61       61           
  Lines        3246     3246           
=======================================
  Hits         2993     2993           
  Misses        253      253           

Flag	Coverage Δ
project	92.20% <ø> (ø)
validator	65.89% <ø> (+0.77%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f79da65...0417216. Read the comment docs.

[CasperWA]

Well you write one thing in the OP and have done another, hehe. I have found this issue in my local installation as well when trying to update the installation I have of optimade[all].
I am surprised this is not caught in our CI tests - I guess it's because the resolver is not applied when installing with -r? Or is it the order of installing the dependencies perhaps?

[CasperWA]

Anyway, I'll approve this for now as we should be able to publish versions at least.
But we should make an issue for this that we should properly test the installation with the newest pip resolver and be as compliant as possible.

[ml-evs]

Ah whoops, I tried the legacy resolver first then switched the installation order instead, but forgot to push... oh well, needs to be fixed anyway. Thanks for the quick review on this @CasperWA.