Change the TimestampStrings check to check for str instead of not ZoneInfo.

This fixes the pytest problem which threw:
TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

during Mocking

src/corelibs/string_handling/timestamp_strings.py

@@ -28,7 +28,7 @@ class TimestampStrings:
         self.time_zone = time_zone if time_zone is not None else self.TIME_ZONE
         try:
             self.timestamp_now_tz = datetime.now(
-                ZoneInfo(self.time_zone) if not isinstance(self.time_zone, ZoneInfo) else self.time_zone
+                ZoneInfo(self.time_zone) if isinstance(self.time_zone, str) else self.time_zone
             )
         except ZoneInfoNotFoundError as e:
             raise ValueError(f'Zone could not be loaded [{self.time_zone}]: {e}') from e