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](https://python-poetry.org/)
- [ ] add badges for tests passing and code quality
- [ ] add contributor guide (include whatever requirements the devs have)