Fix argument type for datetime.now. (#268)

It seems to actually take a tzinfo argument, not its subclass
timezone.

stdlib/3/datetime.pyi

@@ -181,7 +181,7 @@ class datetime:
     @classmethod
     def fromordinal(cls, n: int) -> datetime: ...
     @classmethod
-    def now(cls, tz: timezone = ...) -> datetime: ...
+    def now(cls, tz: _tzinfo = ...) -> datetime: ...
     @classmethod
     def utcnow(cls) -> datetime: ...
     @classmethod