Description of changes:
Changes in 2.0.x depend on behavior introduced in cryptography 2.5, so we need to correctly model our dependency.

Additional testing:
If I set the requirement to ~=2.4.2 we see failures due to how cryptography checks byte types. If I set the requirement to ~=2.5.0, it passes.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.