Remove abc from the abstract base classes.

src/abstract.py

-from abc import ABC, abstractmethod
+"""Collection of abstract classes for type hinting in the rest of the project."""
+# We aren't using the abc module because it isn't available on circuitpy.
 
-
-class ProximitySensor(ABC):
+class ProximitySensor:
     """Abstract proximity sensor."""
 
     @property
-    @abstractmethod
     def proximity(self) -> int:
-        ...
+        return -1
 
 
-class LEDController(ABC):
+class LEDController:
     """Abstract LED controller."""
 
-    @abstractmethod
     def fill(self, color: tuple[int, int, int]) -> None:
-        ...
+        pass
 
 
-class TouchSensor(ABC):
+class TouchSensor:
     """Abstract touch sensor."""
 
     @property
-    @abstractmethod
     def value(self) -> bool:
-        ...
+        return False