fix: remove incorrect skipif and add missing edge case tests

- Remove skipif on test_nanosecond_precision_with_timezone — after
  truncation the string is valid on Python 3.10
- Add test_nanosecond_precision_with_z_suffix (combined Z + truncation)
- Add test_trailing_dot_raises (malformed fractional seconds)
- Remove unused sys import

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dtsong

tests/test_datetime_parsing.py

@@ -1,4 +1,3 @@
-import sys
 from datetime import datetime, timedelta, timezone
 
 import pytest
@@ -43,14 +42,19 @@ def test_z_suffix_utc(self):
         result = _parse_datetime("2022-06-03T12:24:35Z")
         assert result == datetime(2022, 6, 3, 12, 24, 35, tzinfo=timezone.utc)
 
-    @pytest.mark.skipif(
-        sys.version_info < (3, 11), reason="fromisoformat needs 3.11+ for fractional seconds with tz offset"
-    )
     def test_nanosecond_precision_with_timezone(self):
         result = _parse_datetime("2022-06-03T12:24:35.123456789+05:30")
         expected_tz = timezone(timedelta(hours=5, minutes=30))
         assert result == datetime(2022, 6, 3, 12, 24, 35, 123456, tzinfo=expected_tz)
 
+    def test_nanosecond_precision_with_z_suffix(self):
+        result = _parse_datetime("2022-06-03T12:24:35.123456789Z")
+        assert result == datetime(2022, 6, 3, 12, 24, 35, 123456, tzinfo=timezone.utc)
+
+    def test_trailing_dot_raises(self):
+        with pytest.raises(ValueError):
+            _parse_datetime("2022-06-03T12:24:35.")
+
     def test_invalid_string_raises(self):
         with pytest.raises(ValueError):
             _parse_datetime("not-a-date")