Clarify python instructions (#302)
Clarify python instructions (#302)

- Be explicit about python2 requirement
- Remove `sudo` requirement on macOS
- Use `python -m pip` and `python -m virtualenv` to ensure expected python interpreter is running pip/virtualenv
- Tidy formatting
jmhale/algo,trailofbits/algo,trailofbits/algo,jmhale/algo