Fix traceback_timestamps tests for cross-platform line endings

Use regex assertions with multi-line mode to match end of lines instead of
literal newline characters. This allows the tests to pass on Windows which
uses \r\n line endings instead of Unix \n line endings.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: gpshead

Lib/test/test_traceback_timestamps.py

@@ -230,8 +230,8 @@ def test_strip_exc_timestamps_function(self):
                 # Verify original strings have timestamps and stripped ones don't
                 self.assertIn("ZeroDivisionError: division by zero <@", output)
                 self.assertNotIn("ZeroDivisionError: division by zero\n", output)
-                self.assertIn("ZeroDivisionError: division by zero\n", stripped_output)
-                self.assertIn("FakeError: not an exception\n", stripped_output)
+                self.assertRegex(stripped_output, r"(?m)ZeroDivisionError: division by zero$")
+                self.assertRegex(stripped_output, r"(?m)FakeError: not an exception$")
 
     @force_not_colorized
     def test_strip_exc_timestamps_with_disabled_timestamps(self):
@@ -249,10 +249,10 @@ def test_strip_exc_timestamps_with_disabled_timestamps(self):
         stripped_output = result.out.decode() + result.err.decode(errors='ignore')
 
         # All strings should be unchanged by the strip function
-        self.assertIn("ZeroDivisionError: division by zero\n", stripped_output)
+        self.assertRegex(stripped_output, r"(?m)ZeroDivisionError: division by zero$")
         # it fits the pattern but traceback timestamps were disabled to strip_exc_timestamps does nothing.
-        self.assertIn(
-            "FakeError: not an exception <@1234567890.123456>\n", stripped_output
+        self.assertRegex(
+            stripped_output, r"(?m)FakeError: not an exception <@1234567890.123456>$"
         )
 
     def test_timestamp_regex_pattern(self):