Install issues due to uv backend

[cthoyt]

FYI if you are having some install issues due to the uv backend, make sure to set the environment variable UV_PREVIEW=1 before doing pip install -e . or uv pip install -e ..

Right now, I'm trying out this experimental backend to replace setuptools, which should stabilize and not be a "preview" feature in the near future. Then we can return to business as normal with no need to set the environment variable.