Infrastructure things

Looking through the tests, I noticed a few things I figured I could fix. I think JMESPath is a fantastic tool/standard, and I'd like to help out by cleanup and improving CI/CD if the devs think these are useful changes. I can make most of them quite quickly.

• [ ] GitHub workflows could test windows and macos
• [ ] Drop or replace ancient tox config (python 2.6 and nose)
• [ ] Drop past-EOL Python versions 3.6 and 3.7
• [ ] Remove OrderedDict in most places; Python 3.7+ guarantees order
• [ ] Replace sys.maxint (removed in Python 3) with sys.maxsize (guaranteed to exist)
• [ ] Clarify test dependencies
• [ ] Fully unittest with pytest
• [ ] Use of pathlib and encoding='utf8' for better platform independence (also: always close open())
• [ ] Fix license header saying "Copyright (c) 2013 Amazon.com, Inc. or its affiliates. All Rights Reserved". I assume this isn't correct, and that it's MIT-licensed.
• [ ] Run pyupgrade to improve code
• [ ] pre-commit to run pyupgrade, isort, and black to simplify future diffs
• [ ] consider using Poetry
• [ ] add badges for tests passing and code quality
• [ ] add contributor guide (include whatever requirements the devs have)

• [ ] Drop past-EOL Python versions 3.6 and 3.7
• [ ] Run pyupgrade to improve code

These are done in PR #315.