Add missing currentframe to logging module (#2712)

stdlib/2and3/logging/__init__.pyi

@@ -6,7 +6,7 @@ from typing import (
 )
 from string import Template
 from time import struct_time
-from types import TracebackType
+from types import TracebackType, FrameType
 import sys
 import threading
 
@@ -27,6 +27,8 @@ else:
 
 raiseExceptions: bool
 
+def currentframe() -> FrameType: ...
+
 if sys.version_info >= (3,):
     _levelToName = ...  # type: Dict[int, str]
     _nameToLevel = ...  # type: Dict[str, int]