Add deprecation warning to BaseHTTPMiddleware

[adriangb]

Implements #1678

[adriangb]

Idea: if/when #1691 gets merged, we could do something like this (a bit cleaner obviously) so that users can copy-paste the fixed code to migrate:

import inspect

async def dispatch(request, call_next):
    await call_next(request)

def replace(dispatch):
    req_name, call_next_name = inspect.signature(dispatch).parameters.keys()
    source = inspect.getsource(dispatch)
    pattern = f"await {call_next_name}({req_name})"
    source = source.replace(f", {call_next_name}", "")
    new_source = source.replace(pattern, f"yield")
    print(new_source)

This can be included as part of the warning.
Not sure it's a great idea, but it's possible!

[adriangb]

Closing this for now, not enough traction for this to happen anytime soon if ever