⬇️(project) downgrade pytest dependencies

With pytest being upgraded to 7.0.0, importlib.readers breaks the tests.
It is not available until python 3.10. Hence, as we are using Python
3.9, pytest is downgraded to its previous release
  (pytest-dev/pytest#9608).

setup.cfg

@@ -64,7 +64,7 @@ dev =
     mkdocstrings==0.18.0
     pyfakefs==4.5.4
     pylint==2.12.2
-    pytest==7.0.0
+    pytest==6.2.5
     pytest-cov==3.0.0
 ci =
     twine==3.8.0