Fix the need to set PYTHONPATH, edited README

Edit wheel.packages metadata to include namespace package "cuda".
Updated README to remove the work-around of setting PYTHONPATH,
as it is no longer necessary.

python/README.md

@@ -26,7 +26,6 @@ nvbench_DIR=$(pwd)/nvbench_install/lib/cmake CUDACXX=/usr/local/cuda/bin/nvcc pi
 ### Verify that package works
 
 ```
-export PYTHONPATH=$(pwd):${PYTHONPATH}
 python test/run_1.py
 ```
 

python/pyproject.toml

@@ -55,4 +55,5 @@ provider = "scikit_build_core.metadata.setuptools_scm"
 root = ".."
 
 [tool.scikit-build.wheel.packages]
+"cuda" = "cuda"
 "cuda/nvbench" = "cuda/nvbench"