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/config.py

@@ -171,7 +171,8 @@ def get_default_config_dir() -> str:
     return os.path.join(data_dir, CONFIG_DIR_NAME)  # type: ignore
 
 
-def ensure_config_exists(config_dir: str, detect_location: bool = True) -> str:
+def ensure_config_exists(config_dir: str, detect_location: bool = True)\
+        -> Optional[str]:
     """Ensure a configuration file exists in given configuration directory.
 
     Creating a default one if needed.
@@ -187,7 +188,8 @@ def ensure_config_exists(config_dir: str, detect_location: bool = True) -> str:
     return config_path
 
 
-def create_default_config(config_dir, detect_location=True):
+def create_default_config(config_dir: str, detect_location=True)\
+        -> Optional[str]:
     """Create a default configuration file in given configuration directory.
 
     Return path to new config file if success, None if failed.
@@ -286,11 +288,8 @@ async def async_hass_config_yaml(hass):
     return conf
 
 
-def find_config_file(config_dir):
-    """Look in given directory for supported configuration files.
-
-    Async friendly.
-    """
+def find_config_file(config_dir: str) -> Optional[str]:
+    """Look in given directory for supported configuration files."""
     config_path = os.path.join(config_dir, YAML_CONFIG_FILE)
 
     return config_path if os.path.isfile(config_path) else None