install uv with uv

[graingert]

also avoid pip entirely in ci

this works around the issue where pip could not install uv on pypy on windows with a broken cache

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.63156%. Comparing base (0ee5a69) to head (35965ad).
Report is 257 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main       #3131         +/-   ##
===================================================
+ Coverage   99.63003%   99.63156%   +0.00152%     
===================================================
  Files            122         126          +4     
  Lines          18380       18456         +76     
  Branches        1222        1226          +4     
===================================================
+ Hits           18312       18388         +76     
  Misses            47          47                 
  Partials          21          21                 

Files with missing lines	Coverage Δ
src/trio/_tests/tools/test_find_activate_script.py	100.00000% <100.00000%> (ø)
src/trio/_tests/tools/test_find_uv_script_url.py	100.00000% <100.00000%> (ø)
src/trio/_tools/find_activate_script.py	100.00000% <100.00000%> (ø)
src/trio/_tools/find_uv_script_url.py	100.00000% <100.00000%> (ø)

[CoolCat467]

Also, for resolving windows virtual environment issues, I also had that problem while working on #2957, and something like this should help:
https://github.com/CoolCat467/trio/blob/a359dacc2bbaaf24586771d783e3c19b99c30dab/ci.sh#L51
Looking at that, the changes with PYTHON_PATH are not necessary here, not handling pre-installed virtual environment case.

[graingert]

Also, for resolving windows virtual environment issues, I also had that problem while working on #2957, and something like this should help: https://github.com/CoolCat467/trio/blob/a359dacc2bbaaf24586771d783e3c19b99c30dab/ci.sh#L51 Looking at that, the changes with PYTHON_PATH are not necessary here, not handling pre-installed virtual environment case.

I plan to pull the Script Vs bin out of sysconfig.get_path

[A5rocks]

Sorry, I'm still not sold on this as a solution. I think it would be significantly better to just avoid setup-python's cache on PyPy+Windows runs.