More typing (#15449)

## Description:

More typing improvements.

Switch to using `mypy.ini` for flexibility

Add `warn_return_any` check except in `homeassistant.util.yaml` that does typing hacks. Fix some type annotations as resulting from this check and ignore others were fixing is hard.

## Checklist:
  - [x] The code change is tested and works locally.
  - [x] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**

homeassistant/core.py

@@ -106,7 +106,7 @@ class CoreState(enum.Enum):
 
     def __str__(self) -> str:
         """Return the event."""
-        return self.value
+        return self.value  # type: ignore
 
 
 class HomeAssistant(object):
@@ -137,7 +137,7 @@ class HomeAssistant(object):
         # This is a dictionary that any component can store any data on.
         self.data = {}
         self.state = CoreState.not_running
-        self.exit_code = None
+        self.exit_code = 0  # type: int
         self.config_entries = None
 
     @property
@@ -239,7 +239,7 @@ class HomeAssistant(object):
 
         target: target to call.
         """
-        task = self.loop.create_task(target)
+        task = self.loop.create_task(target)  # type: asyncio.tasks.Task
 
         if self._track_task:
             self._pending_tasks.append(task)
@@ -252,7 +252,8 @@ class HomeAssistant(object):
             target: Callable[..., Any],
             *args: Any) -> asyncio.Future:
         """Add an executor job from within the event loop."""
-        task = self.loop.run_in_executor(None, target, *args)
+        task = self.loop.run_in_executor(
+            None, target, *args)  # type: asyncio.Future
 
         # If a task is scheduled
         if self._track_task:
@@ -307,7 +308,7 @@ class HomeAssistant(object):
         """Stop Home Assistant and shuts down all threads."""
         fire_coroutine_threadsafe(self.async_stop(), self.loop)
 
-    async def async_stop(self, exit_code=0) -> None:
+    async def async_stop(self, exit_code: int = 0) -> None:
         """Stop Home Assistant and shuts down all threads.
 
         This method is a coroutine.