TYP: Fix NatType.combine type definition

Author: mroeschke

pandas/_libs/tslibs/nattype.pyi

@@ -102,6 +102,7 @@ class NaTType:
         ambiguous: bool | Literal["raise"] | NaTType = ...,
         nonexistent: TimestampNonexistent = ...,
     ) -> NaTType: ...
+    @classmethod
     def combine(cls, date: date_, time: time_) -> NoReturn: ...
     @property
     def tzinfo(self) -> None: ...