setuptools 36.2.2 does not allow empty 'extras_require' list

[andrewmwhite]

In one of my projects, I'm using the empty extras_require list syntax from https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies, but setuptools is throwing errors under 36.2.2 that weren't there in 36.2.0.

$ cat setup.py
from setuptools import setup

setup(
    name='testemptyextras',
    extras_require={'empty': []}
)

With setuptools==36.2.0:

$ pip install -e '.[empty]'
Obtaining file:///Users/amw/Desktop/setuptools
Installing collected packages: testemptyextras
  Running setup.py develop for testemptyextras
Successfully installed testemptyextras

However, under setuptools==36.2.2:

$ pip install -e '.[empty]'
Obtaining file:///Users/amw/Desktop/setuptools
  testemptyextras 0.0.0 does not provide the extra 'empty'
Installing collected packages: testemptyextras
  Running setup.py develop for testemptyextras
Successfully installed testemptyextras

This results in downstream errors when I try to load related dependencies via pkg_resources:

Traceback (most recent call last):
  File "/apps/python/lib/python2.7/site-packages/stevedore/extension.py", line 163, in _load_plugins
    verify_requirements,
  File "/apps/python/lib/python2.7/site-packages/stevedore/named.py", line 123, in _load_one_plugin
    verify_requirements,
  File "/apps/python/lib/python2.7/site-packages/stevedore/extension.py", line 183, in _load_one_plugin
    ep.require()
  File "/apps/python/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2326, in require
    reqs = self.dist.requires(self.extras)
  File "/apps/python/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2579, in requires
    "%s has no such extra feature %r" % (self, ext)
UnknownExtra: Stethoscope 0.0.1 has no such extra feature 'atlas'