Add a __new__ method to datetime.datetime. (#1417)

Author: rchen152

stdlib/2/datetime.pyi

@@ -143,6 +143,9 @@ class datetime(object):
     def __init__(self, year: int, month: int, day: int, hour: int = ...,
                  minute: int = ..., second: int = ..., microseconds: int = ...,
                  tzinfo: tzinfo = ...) -> None: ...
+    def __new__(cls, year: int, month: int, day: int, hour: int = ...,
+                minute: int = ..., second: int = ..., microseconds: int = ...,
+                tzinfo: tzinfo = ...) -> datetime: ...
 
     @property
     def year(self) -> int: ...