“Black” developer refused for a long time to add option to format code with single quotes with very aggressive manners. Now Django devs didn’t see that option for single quotes and code looks unpleasant.
Some people make the case that it's easier to write single quotes (well, depending on the keyboard format anyway). For keyboards in the US standard you have to hold the Shift key to write a double quote. But the good thing about Black is that you can still write your code using single quote and when you run the command line utility it will fix/normalize the code to use double quotes.
Nowadays I got so used to it that I even write my Python code using double quotes. And looking at Python code using single quotes looks weird/unpleasant for me.
syslog('debug',"Just opened %s for output",filename)
Technically single or double quotes have the exact same meaning in Python. What makes people use single quotes is probably other languages like PHP, Perl and Bash.
I know I've made it a habit to default to single quotes unless I know I need double quotes. So that might be where the habit comes from in the Django project. But it's not actually necessary in python so might as well use the most commonly used type of quote.