Added stub for Python 3's _threading_local (#1462)

stdlib/3/_threading_local.pyi

@@ -0,0 +1,18 @@
+# Source: https://github.com/python/cpython/blob/master/Lib/_threading_local.py
+from typing import Any, Dict, List, Tuple
+from weakref import ReferenceType
+
+__all__: List[str]
+localdict = Dict[Any, Any]
+
+class _localimpl:
+    key: str
+    dicts: Dict[int, Tuple[ReferenceType, localdict]]
+    def __init__(self) -> None: ...
+    def get_dict(self) -> localdict: ...
+    def create_dict(self) -> localdict: ...
+
+class local:
+    def __getattribute__(self, name: str) -> Any: ...
+    def __setattr__(self, name: str, value: Any) -> None: ...
+    def __delattr__(self, name: str) -> None: ...