Fix black --help output for --python-cell-magics option to be reproducible (#3516)

Author: echoix

CHANGES.md

@@ -69,6 +69,8 @@
 
 <!-- Changes to Black's terminal output and error messages -->
 
+- Calling `black --help` multiple times will return the same help contents each time
+  (#3516)
 - Verbose logging now shows the values of `pyproject.toml` configuration variables
   (#3392)
 - Fix false symlink detection messages in verbose output due to using an incorrect

src/black/__init__.py

@@ -244,7 +244,7 @@ def validate_regex(
     multiple=True,
     help=(
         "When processing Jupyter Notebooks, add the given magic to the list"
-        f" of known python-magics ({', '.join(PYTHON_CELL_MAGICS)})."
+        f" of known python-magics ({', '.join(sorted(PYTHON_CELL_MAGICS))})."
         " Useful for formatting cells with custom python magics."
     ),
     default=[],