cuda.core: fix invalid python_version target in pixi.toml (#2029)

Pixi's [package.target.<X>] only accepts platform identifiers; the
'python_version < "3.11"' selector caused every pixi invocation against
cuda_core/pixi.toml to fail at manifest-parse time
('python_version < "3.11"' is not a known platform).

Move backports.strenum into [package.run-dependencies] unconditionally
and let conda-forge's per-Python variants (real impl on <3.11, no-op
stub on >=3.12) handle version selection. Lockfile regenerated.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rparolin

cuda_core/pixi.lock

@@ -185,8 +185,10 @@ cuda-version = "*"
 numpy = "*"
 cuda-bindings = "*"
 cuda-pathfinder = "*"
-
-[package.target.'python_version < "3.11"'.run-dependencies]
+# Conda-forge ships per-Python variants of backports.strenum (real impl on
+# <3.11, no-op stub on >=3.12); pixi-build doesn't accept PEP 508-style
+# `python_version` selectors in [package.target.<X>], so list it
+# unconditionally and let the solver pick the right variant.
 "backports.strenum" = "*"
 
 [target.linux.tasks.build-cython-tests]