Use contextvars API compatible down to python 3.7

asgiref/testing.py

@@ -17,12 +17,13 @@ def __init__(self, application, scope):
         self.scope = scope
         self.input_queue = asyncio.Queue()
         self.output_queue = asyncio.Queue()
-        self.future = asyncio.create_task(
+        # Clear context - this ensures that context vars set in the testing scope
+        # are not "leaked" into the application which would normally begin with
+        # an empty context. In Python >= 3.11 this could also be written as:
+        # asyncio.create_task(..., context=contextvars.Context())
+        self.future = contextvars.Context().run(
+            asyncio.create_task,
             self.application(scope, self.input_queue.get, self.output_queue.put),
-            # Clear context - this ensure that context vars set in the testing scope
-            # are not "leaked" into the application which would normally begin with
-            # an empty context.
-            context=contextvars.Context(),
         )
 
     async def wait(self, timeout=1):