Clean Code: Python

Fix issues found by flake8-bugbear:

* Unused loop variables
* assert statements in non-test code
* mixing test code with production code
* mark one excessive test which takes ~10min to run
  as 'slow'

21_Calendar/python/calendar.py

@@ -109,9 +109,9 @@ def calendar(weekday, leap_year):
         print(days)
         print(sep)
 
-        for w in range(1, 7):
+        for _ in range(1, 7):
             print("\n")
-            for g in range(1, 8):
+            for g in range(1, 8):  # noqa
                 d += 1
                 d2 = d - days_count