Add unittests to mypy check

Summary

Currently the mypy pipeline make mypy only checks src/linkahead. Since the unittest provide actualy code coverage mimicking real-life use of the classes and functions, we should probably also check the types there.

Expected Behavior

unittests are type checked.

Actual Behavior

unittests are not type checked.

Steps to Reproduce the Problem

Specifications

• Version: any
• Platform: any

Possible fixes

Add unittests folder to mypy pipeline. This requires adding a py.typed marker: This is a file that tells type checkers that the library's type annotations are complete and should be used. See https://mypy.readthedocs.io/en/latest/installed_packages.html#creating-pep-561-compatible-packages