Use globals for the database connection

- remove the factory functions for creating engines and sessions
- define global engine, connection, and session objects to be used
  everywhere in the urban_meal_delivery package

setup.cfg

@@ -134,7 +134,7 @@ per-file-ignores =
         WPS432,
     src/urban_meal_delivery/db/__init__.py:
         # Top-level of a sub-packages is intended to import a lot.
-        F401,
+        F401,WPS201,
     tests/*.py:
         # Type annotations are not strictly enforced.
         ANN0, ANN2,