Drop support for Python 3.7

[edmorley]

Since:

• It has been end of life upstream since 2023-06-27, so is no longer receiving security updates.
• The buildpack has been warning about its deprecation/end of life in the build log output since 2023-02-08 (#1404, #1460), and its deprecation was previously announced on changelog: https://devcenter.heroku.com/changelog-items/2530
• The upgrade from Python 3.7 to Python 3.8+ in most cases requires no changes to an application at all (see What’s New In Python 3.8), or else some small dependency updates (compared to say the upgrade from Python 2 to 3).
• Python 3.7 is not compatible with OpenSSL 3, and so only works on the Heroku-20 stack - which itself will be deprecated in 2024.
• Any apps that must stay on Python 3.7 short term, can pin to the previous buildpack version (v238) as a stop-gap.

This also unblocks updating setuptools and pipenv in the buildpack, both of which have dropped support for Python 3.7 in recent versions.

Closes heroku/roadmap#198. GUS-W-12345803.

[edmorley]

https://devcenter.heroku.com/changelog-items/2719

https://devcenter.heroku.com/admin/articles/diff/715/715v179..715v180