Setup pre-commit hooks and reformat code

[asvetlov]

Install hooks: make install

After that, git commit ... runs registered hooks from .pre-commit-config.yaml to committed files.
To skip hooks please use git commit --no-verify ...

The set of hooks can be adjusted, I've applied a configuration that I have for other projects.

[codecov-commenter]

Codecov Report

Merging #245 (e2b0e29) into master (47ae437) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #245   +/-   ##
=======================================
  Coverage   95.59%   95.59%           
=======================================
  Files           2        2           
  Lines         227      227           
=======================================
  Hits          217      217           
  Misses         10       10           

Impacted Files	Coverage Δ
pytest_asyncio/__init__.py	100.00% <ø> (ø)
pytest_asyncio/plugin.py	95.53% <100.00%> (ø)

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