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.
3 jobs for setuptools in 1 minute and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #922807
build-python27

00:01:17

passed #922808
build-python37

00:01:07

 
  Docs
passed #922809
docs

00:00:24