add _levelNames to logging/__init__.py (#1404)

* add _levelNames to logging/__init__.py

* add Python 3's _levelToName and _nameToLevel

Author: matthiaskramm

stdlib/2and3/logging/__init__.pyi

@@ -22,6 +22,11 @@ _Level = Union[int, Text]
 
 raiseExceptions: bool
 
+if sys.version_info >= (3,):
+    _levelToName = ...  # type: Dict[int, str]
+    _nameToLevel = ...  # type: Dict[str, int]
+else:
+    _levelNames = ...  # type: dict
 
 class Logger:
     name = ...  # type: str