add Python 3.12

[minrk]

just ran python3 repo2docker/buildpacks/conda/freeze.py 3.12 and added 3.12 to the refreeze list

closes #1225

[manics]

The latest version of requests breaks docker-py, causing all tests to fail:
docker/docker-py#3256

[minrk]

Not in a rush, let's let that settle for a bit before pinning down requests to avoid a security patch, though this particular case doesn't affect repo2docker users since it only applies to multiple requests to the same host where the value of verify changes from False to True, which is a sequence that can't happen within a run of repo2docker, I think.

[manics]

tests/venv/binder-dir and tests/venv/start/postBuild are failing for an unrelated reason (also failing on main):
KeyError: 'The Python module ipyleaflet is not a valid nbextension, it is missing the _jupyter_nbextension_paths() method.'

[minrk]

ipyleaflet 0.19 refactored some things, and ipyleaflet is no longer a valid extension package (jupyter_leaflet is). I'm not sure if this is intentional, but this command is still in their installation docs, so I'm guessing it is a mistake.

This nbextension step has not been required since notebook 5.3, so these tests (especially postBuild) aren't really testing that they do anything.