docs: improve some $(brew --prefix) usage

[aristocrates]

Intended change: for brew docs https://docs.brew.sh/Homebrew-and-Python#python-3y to say $(brew --prefix python)/libexec/bin instead of $(brew --prefix)/opt/python/libexec/bin in order to be more robust to edge cases from package installation issues.

brew --prefix python errors out when the python package is not installed, so using that instead of brew --prefix prevents python package installation issues from silently breaking, for example, shell env scripts that add this to $PATH.

See https://github.com/orgs/Homebrew/discussions/3596#discussioncomment-5050569

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[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @aristocrates!

I fixed up a few more references in here too. Nothing wrong with your work, just wanted to do this before I forgot.