Remove Python 3.9 port

Python 3.9 is now the minimum supported version, so `is_relative_to` is available.

tests/test_parallel.py

@@ -44,22 +44,10 @@ def test_cache_cleared_by_other_process(
     extract("google.com")
     orig_unlink = os.unlink
 
-    def is_relative_to(path: Path, other_path: str | Path) -> bool:
-        """Return True if path is relative to other_path or False.
-
-        Taken from the Python 3.9 standard library.
-        Reference: https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.is_relative_to
-        """
-        try:
-            path.relative_to(other_path)
-            return True
-        except ValueError:
-            return False
-
     def evil_unlink(filename: str | Path) -> None:
         """Simulate someone deletes the file right before we try to."""
         if (isinstance(filename, str) and filename.startswith(cache_dir)) or (
-            isinstance(filename, Path) and is_relative_to(filename, cache_dir)
+            isinstance(filename, Path) and filename.is_relative_to(cache_dir)
         ):
             orig_unlink(filename)
         orig_unlink(filename)