Run linters in CI

[and-semakin]

What do these changes do?

The project seems to use flake8, mypy, black & isort but doesn't enforce it in the CI. This PR makes it necessary for these tools to pass successfully.

I had to # type: ignore one type errors. At the moment I don't have a clear understanding of how to fix this issue better. The PR still adds some value because all other parts of the project will be type-checked, and no new type violations will be allowed.

Are there changes in behavior for the user?

No. This PR affects only maintainers and future contributors.

Related issue number

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [ ] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
  • .feature: Signifying a new feature.
  • .bugfix: Signifying a bug fix.
  • .doc: Signifying a documentation improvement.
  • .removal: Signifying a deprecation or removal of public API.
  • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov[bot]]

Codecov Report

Merging #868 (a446e02) into master (9048072) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #868   +/-   ##
=======================================
  Coverage   93.36%   93.36%           
=======================================
  Files          11       11           
  Lines        1568     1568           
  Branches      186      186           
=======================================
  Hits         1464     1464           
  Misses         73       73           
  Partials       31       31           

Impacted Files	Coverage Δ
aiopg/utils.py	91.42% <100.00%> (ø)

---

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 9048072...a446e02. Read the comment docs.

[Pliner]

Thanks!