Use mimalloc on all Python 3.13 builds

[zanieb]

Previously, it was only used for the freethreaded builds

Closes #389

[colesbury]

The --with-mimalloc option is the default, so I think 3.13 it's already built with it. The configure option just adds build time support. You need to set the environment variable PYTHONMALLOC=mimalloc to use mimalloc in the default build. (The free threading build requires and always uses mimalloc.)

I ran PYTHONMALLOC=mimalloc uv run -p 3.13 python -c "while True: x = object()" locally and I see mimalloc functions being executed in perf top.

[zanieb]

Thanks for the info! It's the default if it's available, which it should be for most of our builds but it might be nice to make it explicit so it fails if it's not available for some reason. Notably the musl builds don't have a sufficient LLVM version for mimalloc and fail here — this is a known issue, ref #326 (comment).