Change 'dependant' to 'dependent' (#103745)

The word 'dependent' is both an adjective and a noun. A 'dependant' is a British alternative spelling for the noun form. In idlelib.sidebar, 'OS-dependant' is an adjective and clearly wrong. In 'Using', 'dependant' as a noun would be acceptable in Britain, but we use American spellings in Python docs.
https://www.merriam-webster.com/words-at-play/spelling-variants-dependent-vs-dependant