Usually we'll want to set up development environments using

--all-pyvers, which will set up all supported versions. In some cases,

however, it can be useful to set up a subset of versions (for example,

right now Python 3.6 and 3.7 don't work correctly on Apple M1). This

change adds a new argument that allows doing something like

python setup.py develop --pyvers "2.7 3.8 3.9"

Used to set up a dev environment.