Add an isort configuration to sort imports according to Django style guide

[kelseyq]

Django style guide and Two Scoops of Django suggest sorting imports in the following way:

1. Standard library imports
2. Imports from core Django.
3. Imports from third-party apps including those unrelated to Django.
   4. Imports from other Django apps in your project (not sure we can generate anything for this)
4. Local imports

We can use isort to do this sorting automatically. This commit should add an isort dependency in requirements_dev.txt, add an isort section in setup.cfg that sorts the imports correctly, and include instructions in CONTRIBUTING.rst for running isort before contributing.

[EmadMokhtar]

@kelseyq is the setup.cfg for the project itself or for the generated package?

[kelseyq]

@EmadMokhtar I was thinking for the generated package, but we could use both!