Fixed #31224 -- Added support for asynchronous views and middleware.

This implements support for asynchronous views, asynchronous tests,
asynchronous middleware, and an asynchronous test client.

tests/middleware_exceptions/views.py

@@ -27,3 +27,11 @@ def exception_in_render(request):
             raise Exception('Exception in HttpResponse.render()')
 
     return CustomHttpResponse('Error')
+
+
+async def async_exception_in_render(request):
+    class CustomHttpResponse(HttpResponse):
+        async def render(self):
+            raise Exception('Exception in HttpResponse.render()')
+
+    return CustomHttpResponse('Error')