Fix type hint

Sphinx does not understand forwared references without quotes. So autodoc will
fail if the quotes are missing.

Author: AlexanderS

atom/datastructures/sortedmap.pyi

@@ -24,7 +24,7 @@ class sortedmap(Generic[K, V]):
     def keys(self) -> List[K]: ...
     def values(self) -> List[V]: ...
     def items(self) -> List[Tuple[K, V]]: ...
-    def copy(self) -> sortedmap[K, V]: ...
+    def copy(self) -> "sortedmap[K, V]": ...
     def __contains__(self, key: K) -> bool: ...
     def __getitem__(self, key: K) -> V: ...
     def __setitem__(self, key: K, value: V) -> None: ...