HerokuにソースコードをPUSHできない

前提・実現したいこと

自作のDjangoのアプリケーションをHerokuで公開したく以下の記事と参考にすすめたところ
5. 手順(アップロード編)の手順5の段階でつまづいています。git push heroku masterで失敗します。
何か心当たりがあれば教えていただきたいです。

https://qiita.com/frosty/items/66f5dff8fc723387108c

発生している問題・エラーメッセージ

git push heroku masterのコマンドを打ったところ以下のエラーがきます。

(base) mitsuokatomohiro@MacPro hobby_management % git push heroku main
Enumerating objects: 60, done.
Counting objects: 100% (60/60), done.
Delta compression using up to 8 threads
Compressing objects: 100% (55/55), done.
Writing objects: 100% (60/60), 1.80 MiB | 165.00 KiB/s, done.
Total 60 (delta 4), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Building on the Heroku-20 stack
remote: -----> Python app detected
remote:  !     Requested runtime (python-3.6.5) is not available for this stack (heroku-20).
remote:  !     Aborting.  More info: https://devcenter.heroku.com/articles/python-support
remote:  !     Push rejected, failed to compile Python app.
remote: 
remote:  !     Push failed
remote:  !
remote:  ! ## Warning - The same version of this code has already been built: a01ca5e08f9ab6abf66cf8a67b3cba3d57c36eaa
remote:  !
remote:  ! We have detected that you have triggered a build from source code with version a01ca5e08f9ab6abf66cf8a67b3cba3d57c36eaa
remote:  ! at least twice. One common cause of this behavior is attempting to deploy code from a different branch.
remote:  !
remote:  ! If you are developing on a branch and deploying via git you must run:
remote:  !
remote:  !     git push heroku <branchname>:main
remote:  !
remote:  ! This article goes into details on the behavior:
remote:  !   https://devcenter.heroku.com/articles/duplicate-build-version
remote: 
remote: Verifying deploy...
remote: 
remote: !       Push rejected to test-hobbymanagement.
remote: 
To https://git.heroku.com/test-hobbymanagement.git
 ! [remote rejected] main -> main (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/test-hobbymanagement.git'