Dependencies are not listed in install_requires

The setup script needs to list some dependencies in install_requires. I'm not sure what the full list of dependencies is, but it includes at least numpy and six...

$ virtualenv foo
$ source foo/bin/activate
(foo) $ pip install python-ligo-lw
(foo) $ python -c 'from ligo.lw import lsctables
ModuleNotFoundError: No module named 'numpy'
(foo) $ python -c 'from ligo.lw import ligolw'
ModuleNotFoundError: No module named 'six'