Switch from pycryptodomex to cryptography.io

[geofft]

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 :)

[Neustradamus]

@sibson: Have you seen this @geofft PR?

[sibson]

@geofft could you resolve conflicts?

I don't have a strong opinion here, my quick google suggests either is fine but gives cryptography a slight edge due to fewer sharp edges

@pmhahn you most recently updated the dependency here. Do you have an opinion?