Update cryptography dependency

[eloquence]

The cryptography library is currently pinned at version 3.2.1, because 3.3 dropped Python 3.5 support, which is the version of Python included with Ubuntu 16.04.

After the full removal of Xenial support (#5725), we should be in a good position to update the cryptography library as part of the SecureDrop 2.0.0 release. We can consider:

• version 3.3.2, which as of this writing is the last release with security fixes, and which was released prior to the inclusion of Rust code
• the latest stable version at the time of release, which will introduce the Rust build requirement.

[eloquence]

With #5911 merged, this should now be unblocked.

[kushaldas]

For the 2.0.0 release, I suggest to move to the 3.3.2 as we can easily build it from the sources, and then in the next release we can change the build process to use the prebuilt wheels from our workflow/git repository.