Enable CI failures for linting

[ml-evs]

This PR updates the setup.cfg with some flake8 ignores: line length (which is handled by black) and module imports being at the top of a file.

Getting this to pass flake8 required a few tweaks: adding some noqa's and deleting some unused lines.

Closes #269.

[codecov]

Codecov Report

Merging #300 into master will decrease coverage by 0.02%.
The diff coverage is 80.64%.

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
- Coverage   90.05%   90.03%   -0.03%     
==========================================
  Files          54       54              
  Lines        2273     2268       -5     
==========================================
- Hits         2047     2042       -5     
  Misses        226      226              

Flag	Coverage Δ
#unittests	90.03% <80.64%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
optimade/adapters/references/__init__.py	100.00% <ø> (ø)
optimade/adapters/structures/cif.py	100.00% <ø> (ø)
optimade/adapters/structures/pymatgen.py	100.00% <ø> (ø)
optimade/filtertransformers/django.py	91.07% <ø> (-0.31%)	⬇️
optimade/models/optimade_json.py	90.90% <0.00%> (ø)
optimade/models/references.py	97.61% <0.00%> (ø)
optimade/validator/validator.py	74.03% <0.00%> (ø)
optimade/filtertransformers/elasticsearch.py	87.74% <85.71%> (ø)
optimade/adapters/__init__.py	100.00% <100.00%> (ø)
optimade/models/__init__.py	100.00% <100.00%> (ø)
... and 2 more

---

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[CasperWA]

So smooth, so clean, so awesome - thanks @ml-evs! :)