Pin flake8 to be < 5

pytest-flake8 doesn't work with flake8 5, as reported in
pytest-flake8#87.

Until that issue is fixed and we can rely on newer pytest-flake8,
pin flake8.

tox.ini

@@ -7,7 +7,7 @@ deps =
     pytest >= 7.0.1
     pytest-black
     pydicom>=2.1.1
-    flake8
+    flake8<5
     pep8-naming
     pytest-flake8
     mypy