Remove namespace_packages line from setup.py

By making nsnitro a namespace package, pip installation ignores __init__.py, which means importing the module doesn't work as described in the examples. Remove the namespace_packages line as it doesn't add any value.

Existing import behaviour (e.g. `from nsnitro.nsnitro import *` is preserved, while also allowing the documented `from nsnitro import *`, or simply `import nsnitro`.

This closes issue #67.