Updated and expanded the developer setup instructions
Added badges for current PyPI version and number of PyPI downloads
Added python_requires to setup.py. This ensures users don't accidentally install on the wrong version of Python (and prevents the GitHub issues we've all seen where someone accidentally uses pip instead of pip3 on Linux and reports the thing not working...).
See individual commits for details. This PR doesn't necessitate a version bump unless you want to.
Summary
See individual commits for details. This PR doesn't necessitate a version bump unless you want to.