Add type annotations

[youtux]

Add type annotations to the codebase, and add a CI step to run mypy (for now we let it fail, as fixing those failures would require even more time)

[codecov]

Codecov Report

Merging #505 (e378ccb) into master (ddcbeb5) will decrease coverage by 2.71%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
- Coverage   95.40%   92.69%   -2.72%     
==========================================
  Files          48       48              
  Lines        1566     1642      +76     
  Branches      166      176      +10     
==========================================
+ Hits         1494     1522      +28     
- Misses         45       82      +37     
- Partials       27       38      +11     

Impacted Files	Coverage Δ
pytest_bdd/gherkin_terminal_reporter.py	75.71% <61.53%> (-4.94%)	⬇️
pytest_bdd/generation.py	83.48% <63.63%> (-6.21%)	⬇️
pytest_bdd/plugin.py	83.87% <66.66%> (-14.13%)	⬇️
tests/utils.py	84.61% <66.66%> (-15.39%)	⬇️
tests/feature/test_cucumber_json.py	92.59% <71.42%> (-7.41%)	⬇️
pytest_bdd/cucumber_json.py	86.36% <72.72%> (-6.97%)	⬇️
pytest_bdd/reporting.py	78.68% <74.07%> (-9.78%)	⬇️
pytest_bdd/utils.py	87.09% <77.77%> (-5.77%)	⬇️
pytest_bdd/scenario.py	89.53% <84.37%> (-1.49%)	⬇️
pytest_bdd/parsers.py	88.00% <85.71%> (-7.66%)	⬇️
... and 10 more

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