Add several Python 3.8 annotations (#3347)

stdlib/3/gc.pyi

@@ -1,6 +1,7 @@
 # Stubs for gc
 
-from typing import Any, Dict, List, Tuple
+import sys
+from typing import Any, Dict, List, Optional, Tuple
 
 
 DEBUG_COLLECTABLE: int
@@ -16,7 +17,10 @@ def disable() -> None: ...
 def enable() -> None: ...
 def get_count() -> Tuple[int, int, int]: ...
 def get_debug() -> int: ...
-def get_objects() -> List[Any]: ...
+if sys.version_info >= (3, 8):
+    def get_objects(generation: Optional[int] = ...) -> List[Any]: ...
+else:
+    def get_objects() -> List[Any]: ...
 def get_referents(*objs: Any) -> List[Any]: ...
 def get_referrers(*objs: Any) -> List[Any]: ...
 def get_stats() -> List[Dict[str, Any]]: ...