Temporarily run AiiDA tests on Python 3.8 only

[ml-evs]

Following the discussion in #395, and also in actions/setup-python#107, easiest thing for us to do for now is to run the AiiDA tests on Python 3.8 only, then relax the requirement on future versions.

[codecov]

Codecov Report

Merging #400 into master will decrease coverage by 0.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
- Coverage   90.24%   89.62%   -0.62%     
==========================================
  Files          55       55              
  Lines        2408     2459      +51     
==========================================
+ Hits         2173     2204      +31     
- Misses        235      255      +20     

Flag	Coverage Δ
#unittests	89.62% <ø> (-0.62%)	⬇️

Impacted Files	Coverage Δ
optimade/adapters/structures/aiida.py	41.93% <0.00%> (-58.07%)	⬇️
optimade/server/config.py	96.15% <0.00%> (-3.85%)	⬇️
optimade/models/links.py	100.00% <0.00%> (ø)
optimade/adapters/base.py	100.00% <0.00%> (ø)
optimade/models/entries.py	100.00% <0.00%> (ø)
optimade/models/references.py	97.67% <0.00%> (+0.05%)	⬆️
optimade/models/responses.py	97.56% <0.00%> (+0.06%)	⬆️
optimade/filtertransformers/mongo.py	97.15% <0.00%> (+0.12%)	⬆️
optimade/validator/validator.py	74.47% <0.00%> (+0.17%)	⬆️
optimade/models/optimade_json.py	94.18% <0.00%> (+0.21%)	⬆️
... and 6 more

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[ml-evs]

Will use that aforementioned executive privilege to merge this in a few minutes without review... @CasperWA

[CasperWA]

So we couldn't fix to 3.7.7 for python 3.7?

[ml-evs]

So we couldn't fix to 3.7.7 for python 3.7?

We can, but I don't think we want to (we'd have to run coverage on a different version anyway, as gh actions doesn't let you compare versions with patch numbers in them atm...). I'd rather let this be fixed upstream in AiiDA rather than pinning us to an old Python version.