Enable tox unit tests for Python 3.11

[fpgmaas]

Describe the bug

Unit tests for Python 3.11 ran succesfully: https://github.com/fpgmaas/deptry/actions/runs/3020905009

However, when the branch was merged, the same pipeline failed: https://github.com/fpgmaas/deptry/actions/runs/3020926227

      value = _canonicalize_regex.sub("-", name).lower()
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  TypeError: expected string or bytes-like object, got 'NoneType'

Seems related to this issue; pypa/pip#11352 and this issue; python-poetry/poetry#6389

For now, disabled the tox unit tests for Python 3.11. These should be enabled again later.