ci(tests): Add mypy linting

Author: tony

.github/workflows/tests.yml

@@ -84,9 +84,14 @@ jobs:
       - name: Install python dependencies
         run: |
           poetry install -E "test coverage lint"
+
       - name: Lint with flake8
         run: |
           poetry run flake8
+
+      - name: Lint with mypy
+        run: poetry run mypy .
+
       - name: Test with pytest
         continue-on-error: ${{ matrix.tmux-version == 'master' }}
         run: |