git push heroku masterがうまく実行されない。

エラーメッセージから、python-3.6.9やheroku-20のバージョンが合ってないのかと思うのですが、解決策がわからずに困っております。
以下がターミナルのコマンドと実行結果です

% git push heroku master

Enumerating objects: 10, done.
Counting objects: 100% (10/10), done.
Delta compression using up to 8 threads
Compressing objects: 100% (9/9), done.
Writing objects: 100% (10/10), 70.02 MiB | 6.43 MiB/s, done.
Total 10 (delta 0), reused 0 (delta 0), pack-reused 0
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Building on the Heroku-20 stack
remote: -----> Determining which buildpack to use for this app
remote: -----> Python app detected
remote: -----> Using Python version specified in runtime.txt
remote:  !     Requested runtime (python-3.6.9) 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: 6bfu9abc2b882998bca85efcf0000e650c00000f
remote:  !
remote:  ! We have detected that you have triggered a build from source code with version 6bfu9abc2b882998bca85efcf0000e650c00000f
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 tw-bot-0000.
remote: 
To https://git.heroku.com/tw-bot-0000.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/tw-bot-0000.git'

heroku logsのURLから表示されるメッセージです↓

％ heroku logs

-----> Building on the Heroku-20 stack
-----> Determining which buildpack to use for this app
-----> Python app detected
-----> Using Python version specified in runtime.txt
 !     Requested runtime (python-3.6.9) is not available for this stack (heroku-20).
 !     Aborting.  More info: https://devcenter.heroku.com/articles/python-support
 !     Push rejected, failed to compile Python app.
 !     Push failed

同じディレクトリ内にある、runtime.txtファイルでpython-3.6.9を指定しております。
Heroku Python Supportを参考にpython-3.6.15に書き換えたりもしましたが、変化なし。

追加で必要な情報があれば、すぐに確認します。
原因や解決方法がわかる方がいらっしゃいましたら、アドバイスいただけると幸いです。