Cap flake8 version in tests.

Author: domdfcoding

tests/requirements.txt

@@ -4,7 +4,7 @@ consolekit>=1.0.0
 coverage>=5.1
 coverage-pyver-pragma>=0.2.1
 faker>=4.1.2
-flake8>=3.8.4
+flake8<5,>=3.8.4
 funcy>=1.16
 importlib-metadata>=3.6.0
 pandas>=1.0.0; implementation_name == "cpython" and python_version < "3.11"