ci: add flake8-no-pep420

In order to ensure coverage discovery and prevent flaky tooling.

.flake8

@@ -8,7 +8,7 @@ extend-ignore =
     E203,
 per-file-ignores =
     # F401: Module imported by unused (non-implicit modules)
-    __init__.py:F401
+    __init__.py:F401,
 extend-exclude =
     # Frozen and not subject to change in this repo:
     get-poetry.py,

.pre-commit-config.yaml

@@ -33,6 +33,7 @@ repos:
         additional_dependencies: &flake8_deps
           - flake8-bugbear==21.9.2
           - flake8-comprehensions==3.7.0
+          - flake8-no-pep420==1.2.0
           - flake8-typing-imports==1.11.0
 
   - repo: https://github.com/pycqa/isort