I'm not an expert, but I believe just s/python/python2/. Also put it in the documentation as a requirement.
Edit: Hee hee
[aeikum@aeikum howdoi]$ howdoi use python2 on default python3
/usr/bin/env python2.7
Edit: Hee hee
[aeikum@aeikum howdoi]$ howdoi use python2 on default python3
/usr/bin/env python2.7
#!/usr/bin/env python2