## Summary
`pip` uses `sysconfig` for Python 3.10 and later by default; however, it
falls back to `distutils` for earlier Python versions, and distros can
actually tell `pip` to continue falling back to `distutils` via the
`_PIP_USE_SYSCONFIG` variable.
By _always_ using `sysconfig`, we're doing the wrong then when
installing into some system Pythons, e.g., on Debian prior to Python
3.10.
This PR modifies our logic to mirror `pip` exactly, which is what's been
recommended to me as the right thing to do.
Closes https://github.com/astral-sh/uv/issues/2113.
## Test Plan
Most notably, the new Debian tests pass here (which fail on main:
https://github.com/astral-sh/uv/pull/2144).
I also added Pyston as a second stress-test.
`uv --system` is failing in GitHub Actions, because `py --list-paths`
returns all the pre-cached Pythons:
```
-V:3.12 * C:\hostedtoolcache\windows\Python\3.12.2\x64\python.exe
-V:3.12-32 C:\hostedtoolcache\windows\Python\3.12.2\x86\python.exe
-V:3.11 C:\hostedtoolcache\windows\Python\3.11.8\x64\python.exe
-V:3.11-32 C:\hostedtoolcache\windows\Python\3.11.8\x86\python.exe
-V:3.10 C:\hostedtoolcache\windows\Python\3.10.11\x64\python.exe
-V:3.10-32 C:\hostedtoolcache\windows\Python\3.10.11\x86\python.exe
-V:3.9 C:\hostedtoolcache\windows\Python\3.9.13\x64\python.exe
-V:3.9-32 C:\hostedtoolcache\windows\Python\3.9.13\x86\python.exe
-V:3.8 C:\hostedtoolcache\windows\Python\3.8.10\x64\python.exe
-V:3.8-32 C:\hostedtoolcache\windows\Python\3.8.10\x86\python.exe
-V:3.7 C:\hostedtoolcache\windows\Python\3.7.9\x64\python.exe
-V:3.7-32 C:\hostedtoolcache\windows\Python\3.7.9\x86\python.exe
```
So, our default selector returns the first entry here. But none of these
are actually in `PATH` except the one that the user installed via
`actions/setup-python@v5` -- that's the point of the action, that it
puts the correct versions in `PATH`.
It seems to me like we should prioritize `PATH` over `py --list-paths`.
Is there a good reason not to do this?
Closes: https://github.com/astral-sh/uv/issues/2056
## Summary
This is essentially a wrapper around something like `--python $(which
python3)`, but gives users a portable and streamlined way to solve the
common pain point of using `uv` in GitHub Actions or a Docker container.
See: https://github.com/astral-sh/uv/issues/1526.
