fix bug in Timestamp.add (#403)

* fix bug in Timestamp.add

* fix .subtract too

Author: ajtmccarty

infrahub_sdk/timestamp.py

@@ -153,7 +153,7 @@ def add(
         nanoseconds: int = 0,
         disambiguate: Literal["compatible"] = "compatible",
     ) -> Timestamp:
-        return Timestamp(
+        return self.__class__(
             self._obj.add(
                 years=years,
                 months=months,
@@ -183,7 +183,7 @@ def subtract(
         nanoseconds: int = 0,
         disambiguate: Literal["compatible"] = "compatible",
     ) -> Timestamp:
-        return Timestamp(
+        return self.__class__(
             self._obj.subtract(
                 years=years,
                 months=months,