Adding a default extra_require environment

@RonnyPfannschmidt - just out of curiosity, did you get a chance to take another stab at the PEP?

I am interested in trying to help with this, so let me know if there is anything I can do. If you don’t have time to work on this I’d be happy to try and take the lead on the PEP - in that case feel free to share what you had already, or let me know if I should just start with a blank slate. Thanks!