Fix CI config to only deploy once

[wren]

Closes #757

This also does the following:

• All tests run on Python 3.6, 3.7, and 3.8
• Each Python version runs on Linux, Mac and Windows
• Adds caching for Python versions installed by pyenv on mac, and choco on windows (speeds up builds considerably)

Todo (possibly in a separate PR):

• Add caching for poetry install (would shave off another minute or so per test suite)
• Look into why Windows python install is getting 503

Checklist

• The code change is tested and works locally.
• Tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?

[micahellison]

🥇

[lock]

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