[DOC] Clarify Python version compatibility to >=py36

[hectormz]

PR Description

Please describe the changes proposed in the pull request:

• Documentation for users and contributors explicitly state Python 3.6+ compatibility
• black is configured to support python 3.6 & 3.7
• setup.py requires Python 3.6+

This PR resolves #493.

PR Checklist

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1. PR in from a fork off your branch. Do not PR from <your_username>:master, but rather from <your_username>:<branch_name>.

2. If you're not on the contributors list, add yourself to AUTHORS.rst.

Quick Check

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  • pycodestyle checking
  • running the test suite
  • docs build

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Code Changes

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• Ensure that you have added tests.
• Run all tests ($ pytest .) locally on your machine.
  • Check to ensure that test coverage covers the lines of code that you have added.
  • Ensure that all tests pass.

Documentation Changes

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• Build the docs locally.
• View the docs to check that it renders correctly.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[codecov]

Codecov Report

Merging #497 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #497   +/-   ##
=======================================
  Coverage   92.61%   92.61%           
=======================================
  Files           9        9           
  Lines         799      799           
=======================================
  Hits          740      740           
  Misses         59       59