Add py.typed marker for mypy

[pm-incyan]

Description

Added missing py.typed marker for better integration with mypy. Tested that the py.typed marker is included when running poetry build and it is for both wheel and source dist.

Types of Changes

• [ ] Core
• [ ] Bugfix
• [ ] New feature
• [x] Enhancement/optimization
• [ ] Documentation

Issues Fixed or Closed by This PR

Fix #485

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7fe8b52) 87.91% compared to head (a57a9ab) 87.91%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #486 +/- ## ======================================= Coverage 87.91% 87.91% ======================================= Files 37 37 Lines 3170 3170 ======================================= Hits 2787 2787 Misses 383 383 ```

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