Add stub setup.py for editable installations.

In order to run:

    pip install -e .[docs,tests]

We found that you either need to add a stub setup.py like this or be
using a very recent version of pip. In order to streamline the
installation process on more machines with a relaxed pip version
constraint, this stub is added.

Fixes #1639.

setup.py

@@ -0,0 +1,6 @@
+"""
+Stub setup.py to allow editable installations on older pips.
+"""
+from setuptools import setup
+
+setup()