Provide --with-pydebug builds of Python

[nickelpro]

Summary

When developing CPython extensions it is very useful to develop and test on non-optimized, --with-pydebug builds of Python. The set of asserts enabled by such builds provide early warning of a plethora of otherwise tricky to detect bugs, most notably bad reference counting.

Doing so today requires developers to download and compile all relevant versions, and somehow make those versions available to their virtual environment managers (you typically don't want these in PATH). uv could simplify this workflow greatly for most use-cases.

uv already provides a free-threaded variant via the t suffix for freethreaded python, see astral-sh/python-build-standalone#320

The same could be provided for --with-pydebug builds via d suffixed versions

Example

uv python install 3.13d

[zanieb]

Related #11387

Notably this differs from the "I just want debug symbols" use-case cc @geofft

[zanieb]

We were about half-way there on this so I pushed some progress #11520 — will discuss more the state of it there but if you're interested in contributing it could use some help.

[nickelpro]

Ah crap, I promise I tried to search for extant issues. Thanks for the rapid response.

I'll give a look through of the existing work and discuss in the MR if there's anything I think is a problem or can help with.