Bump Flake8 to fix AttributeError: 'EntryPoints' object has no attribute 'get' on Python 3.7

Author: hugovk

requirements-test.txt

@@ -1,7 +1,7 @@
 -r requirements.txt
 
 tox>=3.15.1,<4
-flake8>=3.8.1,<3.9
+flake8>=3.8.1,<6
 pytest>=4.6.10,<5 ; python_version < '3'
 pytest>=5.4.2,<7 ; python_version >= '3'
 coverage>=5.1,<6