test_subscribe: general cleanup and simplify test setups

Replicates graphql/graphql-js@2673bdf

Author: Cito

tests/subscription/test_map_async_iterator.py

@@ -6,9 +6,13 @@
 from graphql.subscription.map_async_iterator import MapAsyncIterator
 
 
-async def anext(iterable):
-    """Return the next item from an async iterator."""
-    return await iterable.__anext__()
+try:  # pragma: nocover
+    anext
+except NameError:  # pragma: nocover (Python < 3.10)
+    # noinspection PyShadowingBuiltins
+    async def anext(iterator):
+        """Return the next item from an async iterator."""
+        return await iterator.__anext__()
 
 
 def describe_map_async_iterator():