Bug#837764: python-pip: Using `--extra-index-url` results in `HTTPError: 404 Client Error: NOT FOUND`

[Fabio Natali]

Source: python-pip
Version: 8.1.2-2
Severity: important

Below a transcript of the command used and the error encountered.

```
$ pip install --index-url=https://pypi.python.org/simple/ 
--extra-index-url=http://pypi.example.com/simple/ 
--trusted-host=pypi.example.com example-app==1.0
Collecting example-app==1.0
  Downloading 
http://pypi.example.com/media/dists/example_app-1.0-py2-none-any.whl (200kB)
    100% |████████████████████████████████| 204kB 340kB/s 
Collecting django-uuidfield>=0.4.0 (from example-app==1.0)
Exception:
Traceback (most recent call last):
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/basecommand.py",
 line 215, in main
    status = self.run(options, args)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/commands/install.py",
 line 328, in run
    wb.build(autobuilding=True)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/wheel.py", 
line 750, in build
    self.requirement_set.prepare_files(self.finder)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/req/req_set.py",
 line 370, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/req/req_set.py",
 line 522, in _prepare_file
    finder, self.upgrade, require_hashes)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/req/req_install.py",
 line 268, in populate_link
    self.link = finder.find_requirement(self, upgrade)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/index.py", 
line 442, in find_requirement
    all_candidates = self.find_all_candidates(req.name)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/index.py", 
line 400, in find_all_candidates
    for page in self._get_pages(url_locations, project_name):
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/index.py", 
line 545, in _get_pages
    page = self._get_page(location)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/index.py", 
line 648, in _get_page
    return HTMLPage.get_page(link, session=self.session)
  File 
"/home/user/.virtualenvs/scope/local/lib/python2.7/site-packages/pip/index.py", 
line 760, in get_page
    resp.raise_for_status()
  File 
"/home/user/.virtualenvs/scope/share/python-wheels/requests-2.10.0-py2.py3-none-any.whl/requests/models.py",
 line 844, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
HTTPError: 404 Client Error: NOT FOUND for url: 
http://pypi.example.com/simple/django-uuidfield/
```

Please note the command works regularly after the following update

```pip install --upgrade --force-reinstall pip```


-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.7.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)