requirements-test: Ensure pinned mypy version is used with tox

djangorestframework-stubs 1.7.0 pin mypy to > 0.950 in its requirements,
thus the mypy pinning to 0.942 in tox.ini is no longer respected and
mypy 0.961 is used when running tox, leading to new typing errors.

So ensure mypy 0.942 is used until we bump mypy version used by swh.