Fix missing return type annotations on constructors with no arguments

As discussed in python/mypy#604, functions
with at least one argument type annotation are allowed to omit the
return type annotation, in which case it defaults to `None`. However,
functions with no arguments cannot have argument type annotations,
so the return type of `None` must be explicitly specified.

tests/components/test_button.py

@@ -10,7 +10,7 @@
 class MockButtonDriver(ButtonInterface):
     """A testing driver for the button component."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.state = False
 
     def set_button_state(self, new_state: bool) -> None:

tests/components/test_gpio_pin.py

@@ -16,7 +16,7 @@
 class MockGPIOPinDriver(GPIOPinInterface):
     """A testing driver for the GPIO pin component."""
 
-    def __init__(self):
+    def __init__(self) -> None:
 
         self.pin_count: int = 10
         self._mode: List[GPIOPinMode] = [

tests/components/test_power_output.py

@@ -8,7 +8,7 @@
 class MockPowerOutputDriver(PowerOutputInterface):
     """A testing driver for power outputs."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         self._enabled = False
 
     def get_power_output_enabled(self, identifier: int) -> bool:

tests/test_stack.py

@@ -14,7 +14,7 @@
 class Robot(BaseRobot):
     """A robot."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         self._env = TestEnvironment