Add ORM models for the pixel grids

- add Grid, Pixel, and AddressPixelAssociation ORM models
- each Grid belongs to a City an is characterized by the side_length
  of all the square Pixels contained in it
- Pixels aggregate Addresses => many-to-many relationship (that is
  modeled with SQLAlchemy's Association Pattern to implement a couple
  of constraints)

setup.cfg

@@ -89,6 +89,8 @@ extend-ignore =
     # Comply with black's style.
     # Source: https://github.com/psf/black/blob/master/docs/compatible_configs.md#flake8
     E203, W503, WPS348,
+    # String constant over-use is checked visually by the programmer.
+    WPS226,
     # Allow underscores in numbers.
     WPS303,
     # f-strings are ok.
@@ -114,8 +116,6 @@ per-file-ignores =
         WPS114,WPS118,
         # Revisions may have too many expressions.
         WPS204,WPS213,
-        # No overuse of string constants (e.g., 'RESTRICT').
-        WPS226,
         # Too many noqa's are ok.
         WPS402,
     noxfile.py:
@@ -125,8 +125,6 @@ per-file-ignores =
         WPS202,
         # TODO (isort): Remove after simplifying the nox session "lint".
         WPS213,
-        # No overuse of string constants (e.g., '--version').
-        WPS226,
         # The noxfile is rather long => allow many noqa's.
         WPS402,
     src/urban_meal_delivery/configuration.py:
@@ -134,13 +132,9 @@ per-file-ignores =
         WPS115,
         # Numbers are normal in config files.
         WPS432,
-    # No real string constant over-use.
-    src/urban_meal_delivery/db/addresses.py:
-        WPS226,
-    src/urban_meal_delivery/db/cities.py:
-        WPS226,
-    src/urban_meal_delivery/db/orders.py:
-        WPS226,
+    src/urban_meal_delivery/db/__init__.py:
+        # Top-level of a sub-packages is intended to import a lot.
+        F401,
     tests/*.py:
         # Type annotations are not strictly enforced.
         ANN0, ANN2,
@@ -158,14 +152,15 @@ per-file-ignores =
         WPS202,WPS204,WPS214,
         # Do not check for Jones complexity in the test suite.
         WPS221,
-        # No overuse of string constants (e.g., '__version__').
-        WPS226,
         # We do not care about the number of "# noqa"s in the test suite.
         WPS402,
         # Allow closures.
         WPS430,
         # Numbers are normal in test cases as expected results.
         WPS432,
+    tests/db/fake_data/__init__.py:
+        # Top-level of a sub-packages is intended to import a lot.
+        F401,WPS201,
 
 # Explicitly set mccabe's maximum complexity to 10 as recommended by
 # Thomas McCabe, the inventor of the McCabe complexity, and the NIST.