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
 ```