Switch from pycryptodomex to cryptography.io

[geofft]

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cryptography.io wraps an existing well-known cryptography library (OpenSSL) instead of implementing routines itself, and it's already a dependency of Twisted, so this reduces the overall number of cryptography libraries needed.

[geofft]

I've done a little bit of unit testing by hand but let me try to make some real connections with this too, since I don't think there's much test coverage for this code. Also I'm open to feedback on whether this is a desirable thing at all.

[geofft]

OK, I've confirmed that this works in both of the cases that are affected, i.e., password auth (with Xtightvnc) and Apple Remote Desktop (with macOS 13.6), so I think I got the code right :)