Add enumera (#378)

Author: fabianhjr

stdlib/3/threading.pyi

@@ -2,7 +2,7 @@
 
 # NOTE: These are incomplete!
 
-from typing import Any, Optional, Callable, TypeVar, Union, Mapping, Sequence
+from typing import Any, Optional, Callable, TypeVar, Union, Mapping, Sequence, List
 
 class Thread:
     name = ...  # type: str
@@ -62,3 +62,8 @@ class Condition:
     def wait_for(self, predicate: Callable[[], _T], timeout: float = ...) -> Union[_T, bool]: ...
     def __enter__(self) -> bool: ...
     def __exit__(self, *args): ...
+
+def current_thread() -> Thread: ...
+def active_count() -> int: ...
+def enumerate() -> List[Thread]: ...
+def main_thread() -> Thread: ...