Remove unused Feature, since it's been removed from upstream setuptools

[blag]

The setuptools project has removed the Feature feature, so trying to import it in setup.py fails, and causes cascading failures for any project that installs this package:

Collecting http_parser (from -r /home/circleci/repo/requirements.txt (line 3))
  Downloading https://files.pythonhosted.org/packages/07/c4/22e3c76c2313c26dd5f84f1205b916ff38ea951aab0c4544b6e2f5920d64/http-parser-0.8.3.tar.gz (83kB)

    100% |████████████████████████████████| 92kB 13.7MB/s 
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-m_29wrbf/http-parser/setup.py", line 19, in <module>
        from setuptools import setup, find_packages, Extension, Feature
    ImportError: cannot import name 'Feature'

This PR removes the Feature import. I'm not sure why it was added to begin with, since it was never actually ever used anywhere.

Related: pypa/setuptools#2017

@benoitc Additionally, is there any way we can get a version uploaded to PyPI? The last version on PyPI is from Aug 30, 2013, and there have been substantial improvements since then.

[benoitc]

thanks!