add link to python docs for cwd context manager

tests/__init__.py

@@ -22,6 +22,9 @@ def _create_cwd_change_generator():
  TemporaryDirectory can also be used without context management, as it is
  cleaned up automatically. Nevertheless, it is neater to use an explicit
  context manager here.
+
+ Also see example in docs:
+ https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager
  """
  # based on the pytest cleandir trick
  original_cwd = os.getcwd()