Mitigate deprecation of aiohttp's @middleware decorator

pyproject.toml

@@ -105,6 +105,9 @@ filterwarnings = [
     # this is mitigated by a try/catch in https://github.com/psf/black/pull/2974/
     # this ignore can be removed when support for aiohttp 3.7 is dropped.
     '''ignore:Decorator `@unittest_run_loop` is no longer needed in aiohttp 3\.8\+:DeprecationWarning''',
+    # this is mitigated by a try/catch in https://github.com/psf/black/pull/3198/
+    # this ignore can be removed when support for aiohttp 3.x is dropped.
+    '''ignore:Middleware decorator is deprecated since 4\.0 and its behaviour is default, you can simply remove this decorator:DeprecationWarning''',
     # this is mitigated by https://github.com/python/cpython/issues/79071 in python 3.8+
     # this ignore can be removed when support for 3.7 is dropped.
     '''ignore:Bare functions are deprecated, use async ones:DeprecationWarning''',

src/blackd/middlewares.py

@@ -1,15 +1,25 @@
-from typing import Awaitable, Callable, Iterable
+from typing import TYPE_CHECKING, Any, Awaitable, Callable, Iterable, TypeVar
 
-from aiohttp.web_middlewares import middleware
 from aiohttp.web_request import Request
 from aiohttp.web_response import StreamResponse
 
+if TYPE_CHECKING:
+    F = TypeVar("F", bound=Callable[..., Any])
+    middleware: Callable[[F], F]
+else:
+    try:
+        from aiohttp.web_middlewares import middleware
+    except ImportError:
+        # @middleware is deprecated and its behaviour is the default since aiohttp 4.0
+        # so if it doesn't exist anymore, define a no-op for forward compatibility.
+        middleware = lambda x: x  # noqa: E731
+
 Handler = Callable[[Request], Awaitable[StreamResponse]]
 Middleware = Callable[[Request, Handler], Awaitable[StreamResponse]]
 
 
 def cors(allow_headers: Iterable[str]) -> Middleware:
-    @middleware  # type: ignore[misc]
+    @middleware
     async def impl(request: Request, handler: Handler) -> StreamResponse:
         is_options = request.method == "OPTIONS"
         is_preflight = is_options and "Access-Control-Request-Method" in request.headers
@@ -32,4 +42,4 @@ async def impl(request: Request, handler: Handler) -> StreamResponse:
 
         return resp
 
-    return impl  # type: ignore[no-any-return]
+    return impl