fix for plugin build dependency

cryptography package related: https://stackoverflow.com/a/75298101/47850

use 'cryptography==3.3.2'

Python3 support for OctoPrint 1.4

This has been tested successfully on

OctoPrint	Python	OctoPi
OctoPrint 1.4.2	Python 3.7.3	OctoPi 0.17.0

Tried to ensure python2 compatibility but was unable to test

This was developed in PR #1