setup.py: pin flake8 due to an incompatibility with pytest-flake8

pytest-flake8 is currently incompatible with 5.x flake8 releases.
This downgrades flake8 to an older version which still works.

Author: igrr

setup.py

@@ -39,6 +39,9 @@
     extras_require={
         'testing': [
             'coverage',
+            # remove flake8 once https://github.com/tholo/pytest-flake8/issues/87 is fixed
+            # and the new version of pytest-flake8 is released
+            'flake8==4.0.1',
             'pytest',
             'pycparser',
             'pytest-flake8',