Fix 3.6 CI build error due to cryptography

[zstumgoren]

Code build and tests succeed for Python versions 3.6-3.9 locally using tox. But running on Travis in CI generates this build error for Python 3.6. Some basic attempts at suggested fixes failed. Have disabled Python 3.6 Travis integration for now in .travis.yml.

Reference:

• https://stackoverflow.com/questions/59441794/error-could-not-build-wheels-for-cryptography-which-use-pep-517-and-cannot-be-i https://stackoverflow.com/questions/59200737/what-does-failed-building-wheel-for-cryptography-for-pyopenssl-in-digitalocean-m

[zstumgoren]

Planning to cut over to GH Actions for CI builds and testing #124