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

test-run/timestamp_strings/timestamp_strings.py

@@ -22,6 +22,9 @@ def main():
     print(f"TZ: {ts.time_zone} -> TS: {ts.timestamp_now_tz}")
     ts = TimestampStrings(ZoneInfo("Europe/Vienna"))
     print(f"TZ: {ts.time_zone} -> TS: {ts.timestamp_now_tz}")
+    custom_tz = 'Europe/Paris'
+    ts = TimestampStrings(time_zone=custom_tz)
+    print(f"TZ: {ts.time_zone} -> TS: {ts.timestamp_now_tz}")
 
 
 if __name__ == "__main__":