Solve all issues detected by PyCharm

- as of September 2021, PyCharm is used to write some of the code
- PyCharm's built-in code styler, linter, and type checker issued
  some warnings that are resolved in this commit
  + spelling mistakes
  + all instance attributes must be specified explicitly
    in a class's __init__() method
    => use `functools.cached_property` for caching
  + make `tuple`s explicit with `(...)`
  + one test failed randomly although everything is ok
    => adjust the fixture's return value (stub for Google Directions API)
  + reformulate SQL so that PyCharm can understand the symbols

src/urban_meal_delivery/forecasts/methods/decomposition.py

@@ -154,8 +154,8 @@ def stl(  # noqa:C901,WPS210,WPS211,WPS231
     else:
         robust = False
 
-    # Initialize R only if necessary as it is tested only in nox's
-    # "ci-tests-slow" session and "ci-tests-fast" should not fail.
+    # Initialize R only if it is actually used.
+    # For example, the nox session "ci-tests-fast" does not use it.
     from urban_meal_delivery import init_r  # noqa:F401,WPS433
 
     # Re-seed R every time it is used to ensure reproducibility.