ligo-common dependency breaks `python setup.py test` for other projects that include namespace packages

Since version 1.1.0, ligo-segments has required ligo-common, which provides a pkg_resources-style namespace package but fails to declare it in setup.py in namespace_packages. For other projects that include subpackages of the ligo namespace package, this breaks the ability to run python setup.py test without first installing. This interferes with CI testing.

Please drop the dependency ligo-common. As a workaround, I am requiring ligo-segments<1.1.0 in my projects (see ligo.skymap!62 (merged).