Run tests against Python 3.9 and add trove classifier

[michael-k]

The Python 3.9 jobs on windows currently fails with

ERROR: Could not build wheels for cryptography which use PEP 517 and cannot be installed directly

cryptography provides abi3 wheels, but they are only available for cryptography>=3 on windows. Wheels for Python 3 probably won't be available for cryptography < 3. Is removing the upper bound worth investigating?

Update:

I've checked the changelog and gave it a try. All the tests pass with cryptography 3.x.