Add reference to NEP 29, make sure we keep pace with support schedule

[ZLLentz]

Expected Behavior

• In the docs and the readme, add a reference to NEP 29
• Make sure the CI always tests for at least the lowest python version (currently, 3.7)
• https://numpy.org/neps/nep-0029-deprecation_policy.html

[klauer]

Documentation needs updating.
Our CI scripts - on the other hand - are shared/centralized. It's then up to https://github.com/pcdshub/pcds-ci-helpers/blob/master/travis/shared_configs/python-tester-conda.yml to test 3.7+ (or the NEP 29-defined minimum supported version)