RFC: Make boltons Python 3.7+ only. #339

Not sure how much explication is necessary here, but it's 2023, what better time to finally jump from 2/3 to just 3?

Off the top of my head:

Pros:

1. Get rid of all the PY2/PY3 conditionals cluttering up the code
2. Generally make contribution easier (see the Py2.7 failure in the initial commit of fix: make copy_function work with keyword-only defaults #336 ). Most potential contributors probably don't have a Python 2 installed.

Cons:

1. Will lose some potential utility for something like ecoutils to detect older Python installations

As for the con, there's always the possibility of using older boltons versions, most of the modules have been pretty stable for a while.