getting "invalid environment marker" when trying to install my python project
The problem ended up being the Heroku "buildpack" that I was using.  I had been using the one from "thenovices" for a long time so that I could use numpy, scipy, etc.  
Sadly, that buildpack specifies an old version of setuptools and python, and those versions were not understanding some of the new instructions (python_version) in the newer setup files for cryptography.
If you're facing this problem, Heroku's advice is to move to Docker-based Heroku, rather than "traditional" Heroku.
I'm trying to set up a beta environment on Heroku for my Django-based project, but when I install I am getting:

error in cryptography setup command: Invalid environment marker:
  python_version < '3'

I've done some googling, and it is suggested that I upgrade setuptools, but I can't figure out how to do that.  (Putting setuptools in requirements.txt gives me a different error message.) 
Sadly, I'm still on Python 2.7, if that matters.