Fetch managed toolchains in uv run

[zanieb]

e.g.

❯ cargo run -q -- run --python 3.8.18 --isolated --with anyio -v -- python3
warning: `uv run` is experimental and may change without warning.
DEBUG Syncing ephemeral environment.
DEBUG Searching for Python 3.8.18 in all sources
DEBUG Searching for managed toolchains at `/Users/zb/Library/Application Support/uv/toolchains`
DEBUG Found CPython 3.12.3 at `/opt/homebrew/bin/python3` (search path)
DEBUG Found CPython 3.9.6 at `/usr/bin/python3` (search path)
DEBUG Found CPython 3.12.3 at `/opt/homebrew/bin/python3` (search path)
DEBUG Requested Python not found, checking for available download...
DEBUG Using registry request timeout of 30s
INFO Fetching requested toolchain...
DEBUG Downloading https://github.com/indygreg/python-build-standalone/releases/download/20240224/cpython-3.8.18%2B20240224-aarch64-apple-darwin-pgo%2Blto-full.tar.zst to temporary location /Users/zb/Library/Application Support/uv/toolchains/.tmpnYw0tc
DEBUG Extracting cpython-3.8.18%2B20240224-aarch64-apple-darwin-pgo%2Blto-full.tar.zst
DEBUG Moving /Users/zb/Library/Application Support/uv/toolchains/.tmpnYw0tc/python to /Users/zb/Library/Application Support/uv/toolchains/cpython-3.8.18-macos-aarch64-none
INFO Ignoring empty directory
DEBUG At least one requirement is not satisfied: anyio
...
Installed 5 packages in 19ms
 + anyio==4.4.0
 + exceptiongroup==1.2.1
 + idna==3.7
 + sniffio==1.3.1
 + typing-extensions==4.12.2
DEBUG Running `python3`
Python 3.8.18 (default, Feb 25 2024, 04:25:37) 
[Clang 17.0.6 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

[zanieb]

Requires --with <blah> right now because we don't check if the Python request is satisfied in the project environment? Will investigate that.

[charliermarsh]

Are you looking for #3945?

[zanieb]

@charliermarsh I bet I am! Thought I'd seen something about that :) Thanks!