requested runtime (python-3.7.6) is not available for this stack (heroku-20).
i thimk Python 3.7.1 is not supported by Heroku, as per their documentation on supported runtimes. Change the Python version to 3.7.2 if you want Python 3.7, or 3.6.8 if you don't.
For information on specifying a runtime, see this page of the documentation. By default it uses 3.6.8 for Python, so if you want 3.7.2 you need to create a file called runtime.txt in the root directory of your app with python-3.7.2 written inside.