Add argon2 to crypotgraphy step

- Add argon2-cffi
- Add django[argon2] option from Django docs:
  https://docs.djangoproject.com/en/1.10/topics/auth/passwords/#argon2-usage

This reverts commit 75870dd4, reversing
changes made to 0cf2e284.

Update setuptools

Preserve environment variables

Preserve LIBRARY_PATH and LD_LIBRARY_PATH.
Related heroku/heroku-buildpack-python#287