This adds and improves the string type checks for the crypto and SSH

functions, ensuring we catch invalid input before hitting a cryptic

error message. It also fixes a couple of compatibility issues with

Python 3, involving division and an improper decode on a Unicode string.
The bulk of the change improves test coverage of these functions,

ensuring that all the types match what we expect. As part of this, the

method used to generate SSH keys for the tests have changed to create

stable keys for testing (so we can ensure that fingerprints don't change

across test runs) and to do so in a way that ensures we only load the

keys on first use, reusing them for any future tests.

Unit tests pass on Python 2.7/Django 1.6 and Python 3.7/Django 1.11

(with other in-progress changes).