You can create a file named secret_settings.py and place your SECRET_KEY inside this file. Then add this file to .gitignore.
Then in your settings, you can remove the secret key variable and import it from there. This should ensure that SECRET_KEY variable remains out of version control.
Create a file named secret_settings and then place your SECRET_KEY and other secret settings in it.
SECRET_KEY = .. # add your setting here
Then in your settings.py file, import these settings.
from secret_settings import *
Finally, add secret_settings.py to your .gitignore file.
Note:
If you already have committed some sensitive data to your repo, then change it!
As per Github website in the removing sensitive data article:
If you committed a password, change it! If you committed a key,
generate a new one.
Check this link on how to purge a file from your repository's history.