python CI: use uv backend for nox (#11094)

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Addresses #11092, maybe. 

A speculative fix, guessing that uv won't take as much disk space for
the "reinstall packages" operation because of its solver and caching.

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Python CI should be green, notably the "Test Minimum Positron IPyKernel
Dependencies" step. That problem was intermittent so it might take some
time to have full confidence that this worked.

Author: austin3dickey

extensions/positron-python/noxfile-positron.py

@@ -7,7 +7,7 @@
 import nox
 
 
-@nox.session()
+@nox.session(venv_backend="uv")
 @nox.parametrize("pandas", ["1.5.3"])
 @nox.parametrize("numpy", ["1.24.4"])
 @nox.parametrize("torch", ["1.12.1"])