De-duplicate and sort required packages

[hugovk]

Pillow has a number of optional dependencies ("extras"), and some of them duplicate a package (defusedxml, olefile):

https://github.com/python-pillow/Pillow/blob/f8160b858aebf8b7bffdefba48735feac300a060/pyproject.toml#L41-L73

This means we get duplicates when running pispy pillow:

This PR de-duplicates them by putting them in a set first, and we might as well sort them too:

[davep]

Thanks, although I suspect what you've done here is a portion of what I'm intending to do when I address #20 (probably in a stream on Sunday or Monday).

[hugovk]

That sounds even better, feel free to close this :)

[davep]

I'll keep it open for now as a very obvious note to self that this needs doing. 😁

[davep]

Merging this in as there's a couple of other things I think need looking into when it comes to #20, and this makes for a far cleaner display meanwhile.