Future dependency upgrades: drop Python 3.7, add Python 3.12

[tw4l]

In #839, we decided to continue supporting Python 3.7 for the next release, which resulted in pinning a few older dependencies.

When we're ready for the next release following to drop Python 3.7 and add support for Python 3.12, we'll want to:

• Upgrade werkzeug (will require removing spaces in some tests, due to werkzeug >2.2.3 removing spaces in bodies
• Upgrade gevent (to 23.9.0 if the PSF supported httpbin still requires greenlet< 3, otherwise to latest and greenlet 3)
• Handle deprecation notices from tox