pip: Behavior change between 8.1.2 and 9.0.1 of pip breaks --extra-index-url in requirements files

• Pip version: 9.0.1
• Python version: Python 2.7.13
• Operating system: Debian Stretch

Description:

When moving from Debian Jesse to Stretch, the newer version of pip gets picked up, and ends up breaking our docker build. Our requirements.txt file contains the following, and worked successfully when using pip version 8.1.2

grpcio

--extra-index-url <SANITIZED>
wrigley==0.0.4
junos-eznc
python-consul
retry

Moving to 9.0.1 results in the following error

Collecting wrigley==0.0.4 (from -r requirements.txt (line 4))
Exception:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
  File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 342, in run
    requirement_set.prepare_files(finder)
  File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 380, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 554, in _prepare_file
    require_hashes
  File "/usr/lib/python2.7/dist-packages/pip/req/req_install.py", line 278, in populate_link
    self.link = finder.find_requirement(self, upgrade)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 465, in find_requirement
    all_candidates = self.find_all_candidates(req.name)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 423, in find_all_candidates
    for page in self._get_pages(url_locations, project_name):
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 568, in _get_pages
    page = self._get_page(location)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 683, in _get_page
    return HTMLPage.get_page(link, session=self.session)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 795, in get_page
    resp.raise_for_status()
  File "/usr/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/models.py", line 893, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
HTTPError: 404 Client Error: Not Found for url: https://pypi.python.org/simple/wrigley/