PyCrypto's been unmaintained for a while, which in theory opens the door

to security issues. In practice, this doesn't truly affect us, as we

only use the AES encryption/decryption, which doesn't appear to have had

any security problems. However, from a user standpoint, it's easier to

install the new cryptography module and harder to install PyCrypto, and

it's better to have more modern options.
This change moves us over from PyCrypto fully to cryptography, updating

dependencies accordingly. This is fully backwards-compatible, meaning

that existing installs with encrypted content from PyCrypto will

continue to work with cryptography, and vice-versa if downgrading.
Documentation for the functions have also been added, helping to

clarify the types of encryption used.
Unit tests were added to ensure these operations continue to work and

that backwards-compatibility does not break.

Tested that all credentials on my development server made from PyCrypto

were properly loaded, and new credentials could be saved and then loaded

again.
Unit tests pass.