remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Python app detected
remote:  !     Requested runtime (Python 3.6.4) is not available for this stack (heroku-18).
remote:  !     Aborting.  More info: https://devcenter.heroku.com/articles/python-support
remote:  !     Push rejected, failed to compile Python app.

remote:
remote:  !     Push failed
remote: Verifying deploy...
remote:
remote: !       Push rejected to sreeman-billing.
remote:
To https://git.heroku.com/sreeman-billing.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/sreeman-billing.git'