Most Django sites probably aren't public github projects though
These are more likely Django apps... it'd be interesting to consider how many of them shouldn't even be mentioning Django at all in their requirements.txt files to avoid clashing with the Django version of the project you're importing their app into.
This gets to the setup.py/requirements.txt confusion.
setup.py is for specifying dependencies of a piece of code you're distributing, while requirements.txt is for specifying a known-good environment to run a piece of code in.
For example, my own personal site has a requirements.txt specifying Django 1.11.2 because that's the version I'm deploying on and test it on. But it uses several applications I've written and distribute, and those use setup.py to specify a dependency on Django 1.8, 1.10 or 1.11 (and those all use tox/Travis to test on the full matrix of Python and Django versions they support).
These are more likely Django apps... it'd be interesting to consider how many of them shouldn't even be mentioning Django at all in their requirements.txt files to avoid clashing with the Django version of the project you're importing their app into.