Add py.typed file

[kasium]

This change adds the py.typed file, so that mypy and other type checkers can pickup the types.
Therefore a small tweak to setup.cfg is needed. I added as well the typed classifier for pypi.

I manually verified that the file is present in the distributions

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

Relates #384

❓ What is the current behavior? (You can also link to an open issue here)

❓ What is the new behavior (if this is a feature change)?

📋 Other information:

📋 Contribution checklist:

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this guide on making great pull requests)

• I wrote descriptive pull request text above
• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after
  the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

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This change is 

[codecov]

Codecov Report

Merging #406 (2849ed9) into master (d758451) will decrease coverage by 0.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   80.50%   79.81%   -0.69%     
==========================================
  Files          28       28              
  Lines        4390     4390              
==========================================
- Hits         3534     3504      -30     
- Misses        856      886      +30     

[webknjaz]

I manually verified that the file is present in the distributions

Yep, the thing that automatically includes all of the Git-tracked files into the sdist is setuptools-scm.

[webknjaz]

Thanks!