Fix stable sort in heapq.merge: python uses both __eq__ and __lt__ to… (#112)

* Fix stable sort in heapq.merge: python uses both __eq__ and __lt__ to compare tuples.

Co-authored-by: Yakov Galka <[email protected]>

Author: ybungalobill

asyncstdlib/heapq.py

@@ -99,6 +99,9 @@ async def pull_head(self) -> bool:
     def __lt__(self, other: "_KeyIter[LT]") -> bool:
         return self.reverse ^ (self.head_key < other.head_key)
 
+    def __eq__(self, other: "_KeyIter[LT]") -> bool:  # type: ignore[override]
+        return not (self.head_key < other.head_key or other.head_key < self.head_key)
+
 
 @overload
 def merge(