fix: upgrade Pytest 6.2.2 → 6.2.4

fix: upgrade Pytest 6.2.2 → 6.2.4

Python 3.10 was released on 2021-10-04 with what appears to be a breaking
change.¹ Graphviz’ Windows CI tasks (intentionally) install an unpinned
version of Python 3, so they get whatever the latest Python 3 version available
on Chocolatey is. Python 3.10 became available in Chocolatey on 2021-10-05 and
Windows CI tasks immediately started failing. The root cause is that 3.10
exposes a latent bug in Pytest.² This bug is reportedly fixed in Pytest 6.2.4,
so upgrading should repair Windows CI.

¹ https://bugs.python.org/issue43798
² https://github.com/pytest-dev/pytest/issues/8539