Switch from distutils to setuptools to fix namespace package issues

Distutils and setuptools projects that populate the same namespace package do not play well together. Setuptools is smart enough to skip installation of the namespace package's dummy __init__.py file and its ancillary .pyc or __pycache__ files, but distutils is not. Distutils projects that use namespace packages are likely to disrupt package managers that protect against two packages trying to install the same files.

It is relatively rare these days to find Python projects that use distutils directly rather than setuptools. In fact, the official Python standard library documentation for distutils (https://docs.python.org/3/library/distutils.html) says:

Most Python users will not want to use this module directly, but instead use the cross-version tools maintained by the Python Packaging Authority.

Switching python-ligo-lw from distutils to setuptools simplifies software packaging.