packse has the ability to specify a project wide Requires-Python
constraint, but our lock template wasn't forwarding this to the
corresponding pyproject.toml. This update makes that happen.
This commit adds a template and does some light surgery on `generate.py`
to make use of that template. In particular, the universal tests require
using the "workspace"-aware version of `uv`, so we can't use the
existing `uv pip {compile,install}` tests.