Use markdown-it-py instead of commonmark in the release script. (#18637)

`commonmark` has been deprecated in favor of `markdown-it-py`, and its
type hints have been [removed from
typeshed](python/typeshed#13725).

This switches the release script to use `markdown-it-py` instead of
`commonmark` for parsing the `CHANGES.md`

Author: sandhose

changelog.d/18637.misc

@@ -0,0 +1 @@
+Use `markdown-it-py` instead of `commonmark` in the release script.