Move to `uv` package manager.

[bpblanken]

I'd held off because it's not a functionality improvement, but it's light speed fast and is an immediate noticeable improvement:

uv add openai
Resolved 54 packages in 379ms
Prepared 13 packages in 354ms
Installed 13 packages in 35ms
 + annotated-types==0.7.0
 + anyio==4.11.0
 + distro==1.9.0
 + h11==0.16.0
 + httpcore==1.0.9
 + httpx==0.28.1
 + jiter==0.11.1
 + openai==2.7.1
 + pydantic==2.12.4
 + pydantic-core==2.41.5
 + sniffio==1.3.1
 + tqdm==4.67.1
 + typing-inspection==0.4.2