Document developer environment setup

[grahamalama]

This was my first time using tox, so it took me a while to get up to speed to start making changes. It would be nice to have some documentation for "here's how you get a development environment set up, make a change, and run the tests". Even better if it was in a Makefile or bash script.

[jwhitlock]

my quick notes for macOS:

brew install redis pyenv
# Run `redis-server` in a different terminal or as a system service

# create a virtualenv by favorite method
pyenv install 3.7 3.8 3.9 3.10 3.11  # Install all the supported Pythons
pyenv local 3.7 3.8 3.9 3.10 3.11  # Make them available in the current session
python -m pip install --upgrade tox
ulimit -n 2560 # tox opens more than the default of 256 for me...
tox  # run all tests
tox -l  # list test targets
tox -e py39-dj32  # Run one test target