CI: mypy is not executed on Python 3.10, 3.11, and 3.12

[Mulugruntz]

The culprit seems to be

    - name: Check with mypy
      if: ${{ !startsWith(matrix.python-version, 'pypy') && matrix.python-version >= '3.7' }}
      run: mypy

The comparison is string to string. Therefore 3.10 < 3.7 (1<7).

The version comparison can be entirely removed, as we don't support < 3.7 anymore.

    - name: Check with mypy
-      if: ${{ !startsWith(matrix.python-version, 'pypy') && matrix.python-version >= '3.7' }}
+      if: ${{ !startsWith(matrix.python-version, 'pypy') }}
      run: mypy