Drop support for Python 3.7

[ml-evs]

Closes #1148.

Should not be merged until we are about to release v0.19.

[codecov]

Codecov Report

Merging #1179 (a201609) into master (94704f9) will decrease coverage by 0.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1179      +/-   ##
==========================================
- Coverage   91.16%   90.70%   -0.46%     
==========================================
  Files          72       72              
  Lines        4311     4315       +4     
==========================================
- Hits         3930     3914      -16     
- Misses        381      401      +20     

Flag	Coverage Δ
project	90.70% <ø> (-0.46%)	⬇️
validator	90.70% <ø> (+0.14%)	⬆️

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Impacted Files	Coverage Δ
optimade/__init__.py	100.00% <ø> (+42.85%)	⬆️
optimade/client/client.py	69.51% <0.00%> (-8.97%)	⬇️
optimade/client/cli.py	83.67% <0.00%> (-4.09%)	⬇️
optimade/server/schemas.py	100.00% <0.00%> (ø)
optimade/models/structures.py	95.75% <0.00%> (ø)
optimade/models/utils.py	92.78% <0.00%> (+0.07%)	⬆️
optimade/server/config.py	91.66% <0.00%> (+0.08%)	⬆️
optimade/filtertransformers/elasticsearch.py	85.94% <0.00%> (+1.37%)	⬆️

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