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

tests/unit/string_handling/test_timestamp_strings.py

@@ -3,7 +3,7 @@ PyTest: string_handling/timestamp_strings
 """
 
 from datetime import datetime
-from unittest.mock import patch, MagicMock
+from unittest.mock import patch
 from zoneinfo import ZoneInfo
 import pytest
 
@@ -119,18 +119,55 @@ class TestTimestampStrings:
             assert ts2.time_zone == 'Europe/London'
             assert ts1.time_zone != ts2.time_zone
 
-    @patch('corelibs.string_handling.timestamp_strings.ZoneInfo')
-    def test_zoneinfo_called_correctly(self, mock_zoneinfo: MagicMock):
-        """Test that ZoneInfo is called with correct timezone"""
+    def test_zoneinfo_called_correctly_with_string(self):
+        """Test that ZoneInfo is called with correct timezone when passing string"""
+        with patch('corelibs.string_handling.timestamp_strings.ZoneInfo') as mock_zoneinfo:
+            with patch('corelibs.string_handling.timestamp_strings.datetime') as mock_datetime:
+                mock_now = datetime(2023, 12, 25, 15, 30, 45)
+                mock_datetime.now.return_value = mock_now
+
+                custom_tz = 'Europe/Paris'
+                ts = TimestampStrings(time_zone=custom_tz)
+                assert ts.time_zone == custom_tz
+
+                mock_zoneinfo.assert_called_with(custom_tz)
+
+    def test_zoneinfo_object_parameter(self):
+        """Test that ZoneInfo objects can be passed directly as timezone parameter"""
         with patch('corelibs.string_handling.timestamp_strings.datetime') as mock_datetime:
             mock_now = datetime(2023, 12, 25, 15, 30, 45)
-            mock_datetime.now.return_value = mock_now
+            mock_now_tz = datetime(2023, 12, 25, 15, 30, 45, tzinfo=ZoneInfo('Europe/Paris'))
+            mock_datetime.now.side_effect = [mock_now, mock_now_tz]
 
-            custom_tz = 'Europe/Paris'
-            ts = TimestampStrings(time_zone=custom_tz)
-            assert ts.time_zone == custom_tz
+            # Create a ZoneInfo object
+            custom_tz_obj = ZoneInfo('Europe/Paris')
+            ts = TimestampStrings(time_zone=custom_tz_obj)
 
-            mock_zoneinfo.assert_called_with(custom_tz)
+            # The time_zone should be the ZoneInfo object itself
+            assert ts.time_zone is custom_tz_obj
+            assert isinstance(ts.time_zone, ZoneInfo)
+
+    def test_zoneinfo_object_vs_string_equivalence(self):
+        """Test that ZoneInfo object and string produce equivalent results"""
+        with patch('corelibs.string_handling.timestamp_strings.datetime') as mock_datetime:
+            mock_now = datetime(2023, 12, 25, 15, 30, 45)
+            mock_now_tz = datetime(2023, 12, 25, 15, 30, 45, tzinfo=ZoneInfo('Europe/Paris'))
+            mock_datetime.now.side_effect = [mock_now, mock_now_tz, mock_now, mock_now_tz]
+
+            # Test with string
+            ts_string = TimestampStrings(time_zone='Europe/Paris')
+
+            # Test with ZoneInfo object
+            ts_zoneinfo = TimestampStrings(time_zone=ZoneInfo('Europe/Paris'))
+
+            # Both should produce the same timestamp formats (though time_zone attributes will differ)
+            assert ts_string.today == ts_zoneinfo.today
+            assert ts_string.timestamp == ts_zoneinfo.timestamp
+            assert ts_string.timestamp_file == ts_zoneinfo.timestamp_file
+
+            # The time_zone attributes will be different types but represent the same timezone
+            assert str(ts_string.time_zone) == 'Europe/Paris'
+            assert isinstance(ts_zoneinfo.time_zone, ZoneInfo)
 
     def test_edge_case_midnight(self):
         """Test timestamp formatting at midnight"""