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

My worry is that not everyone will have python2.7 so I'd rather stick to "#!/usr/bin/env python". It would be great if this works but it doesn't

#!/usr/bin/env python2