fix ci configuration

[bmos]

Goals

• Update CircleCI configuration for better maintainability and more complete testing of supported python versions.
• Only change program and test code by adjusting formatting or adding noqa comments to get the checks passing.

What is this change?

• Refactor CI workflow and tooling configurations so that all supported versions of python are tested successfully.
• Remove .egg files from the pypi upload process as they're no longer accepted.
• Move documentation generation to a discrete workflow as it's not code validation.

Considerations for discussion

• Tests are only run if syntax validation passes. Would it be preferable to run testing regardless?
• Python 3.4 testing has been removed as it's not supported by CircleCI. Is this important to have?
• Checksum of tests/requirements-syntax.txt has been added to the CI cache keys.
• Each workflow runner instance runs a single version of python (looking at the previous logs, some commands were running on different versions). Is this ok? It seemed preferable.
• Commands are mostly run via pipenv, but we don't have a lockfile. Why?
• Pytest is configured to suppress any CryptographyDeprecationWarning, is this acceptable?
• Moved flake8 config from tox.ini to config.cfg so it was available to CI runs, appveyor, ect./ Is this ok?

How to test the changes (if needed)

• View CircleCI and Appveyor logs.

[coveralls]

coverage: 91.654% (-0.01%) from 91.667%
when pulling 0f02dc8 on bmos:patch-2
into dc07328 on pahaz:master.

[bmos]

@pahaz When you have a chance can you give this a look-over? I think I have resolved every CI issue :)