Improve type annotations in `routes.py`

[amotl]

@coderabbitai mentioned at #560 (comment):

It appears that the type annotations I suggested for apps are causing mypy to raise errors because app can be either an ASGI application (awaitable) or a WSGI application (non-awaitable). This leads to type inconsistencies when attempting to await app(scope, receive, send).

To address this, we can adjust the type annotations to accurately reflect the possible types of app and modify the code to handle both cases explicitly. Here's a revised approach:

1. Update the type annotations:

   from typing import Union, Callable, Awaitable, Any
   
   ASGIApp = Callable[[dict, Callable, Callable], Awaitable[None]]
   WSGIApp = Callable[..., Any]  # WSGI applications accept 'environ' and 'start_response'
   
   apps: t.Dict[str, Union[ASGIApp, WSGIApp]]

2. Modify the __call__ method to handle both ASGI and WSGI apps:

   async def __call__(self, scope, receive, send):
       # ... existing code ...
   
       # Call into a submounted app, if one exists.
       for path_prefix, app in self.apps.items():
           if path.startswith(path_prefix):
               scope["path"] = path[len(path_prefix) :]
               scope["root_path"] = root_path + path_prefix
               if asyncio.iscoroutinefunction(app):
                   # ASGI application
                   await app(scope, receive, send)
               else:
                   # WSGI application
                   app = WSGIMiddleware(app)
                   await app(scope, receive, send)
               return
   
       # ... existing code ...

This way, we explicitly check if app is an awaitable ASGI application or a regular WSGI application and handle each case accordingly. This should resolve the mypy errors by ensuring type consistency.

Let me know if this helps or if you have further questions.

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Originally posted by @coderabbitai[bot] in #560 (comment)