rename PromiseCanceller to AbortSignalListener

GraphQL-Core has no PromiseCanceller class. Its cancellation mechanism
is the with_abort_signal helper, so this only updates the renamed
upstream symbol references in the cancellable_iterable comment and
renames the abort-signal test file to test_cancellation.py for parity
with the renamed cancellation-test.ts.

Replicates graphql/graphql-js@c65bd81

Author: Cito

src/graphql/execution/execute.py

@@ -969,9 +969,9 @@ def cancellable_iterable(self, iterable: AsyncIterable[T]) -> AsyncIterable[T]:
 
         When the abort signal is triggered, any pending ``__anext__`` call returns
         immediately by raising the abort reason. This mirrors the JavaScript
-        ``PromiseCanceller.cancellableIterable``; GraphQL-Core needs no
-        ``PromiseCanceller`` class since :meth:`with_abort_signal` already provides
-        the cancellation mechanism.
+        ``cancellableIterable``; GraphQL-Core needs no ``AbortSignalListener``
+        class since :meth:`with_abort_signal` already provides the cancellation
+        mechanism.
         """
         if self.abort_signal is None:
             return iterable