Add flake8 and mypy linting

[blink1073]

No description provided.

[codecov]

Codecov Report

Merging #46 (76601b2) into master (2190017) will decrease coverage by 0.31%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
- Coverage   90.72%   90.41%   -0.32%     
==========================================
  Files           5        5              
  Lines         291      292       +1     
==========================================
  Hits          264      264              
- Misses         27       28       +1     

Impacted Files	Coverage Δ
pytest_check_links/plugin.py	92.46% <62.50%> (-0.37%)	⬇️
pytest_check_links/__init__.py	100.00% <100.00%> (ø)
pytest_check_links/args.py	96.66% <100.00%> (ø)

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[blink1073]

I'll cut a new release once this goes in.

[bollwyvl]

LGTM!