Impossible de mettre à jour pip derrière un proxy

J'ai essayé toutes les suggestions de cette autre question, mais cela ne fonctionne pas pour moi : Impossible de mettre à jour pip

Je suis sous Ubuntu 12.04, et j'ai pu installer pip 1.0 en utilisant apt mais cette version a apparemment un bug qui ne fonctionne pas avec les proxies, donc je ne sais pas comment la mettre à jour.

Voici ce que j'obtiens dans ma console :

$ pip --version
pip 1.0 from /usr/lib/python2.7/dist-packages (python 2.7)
$ which pip
/usr/local/bin/pip

essai 1

$ pip --proxy=$http_proxy install -U pip
Downloading/unpacking pip
  Cannot fetch index base URL http://pypi.python.org/simple/
  Could not find any downloads that satisfy the requirement pip
No distributions at all found for pip
Storing complete log in /home/falconer_k/.pip/pip.log

Essai n°2

$ python ~/Downloads/get-pip.py --proxy=$http_proxy
/usr/lib/python2.7/dist-packages/setuptools/command/install_scripts.py:3:
UserWarning: Module pip was already imported from 
/tmp/tmpqbetIm/pip.zip/pip/__init__.py, but /usr/lib/python2.7/dist-packages
is being added to sys.path from pkg_resources import Distribution, PathMetadata, ensure_directory
Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after 
connection broken by 'ProxyError('Cannot connect to proxy.', 
NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.Verified
HTTPSConnection object at 0x392eed0>: Failed to establish a new connection: 
[Errno -2] Name or service not known',))': /simple/pip/

Je peux accéder au simple/pip directement à l'aide du navigateur :

https://pypi.Python.org/simple/pip/

Je n'ai pas non plus la possibilité d'utiliser une connexion non proxy.

Voici le contenu de ~/.pip/pip.log :

------------------------------------------------------------
/usr/bin/pip run on Tue May  3 15:33:22 2016
Downloading/unpacking pip
  Getting page http://pypi.python.org/simple/pip
  Could not fetch URL http://pypi.python.org/simple/pip: timed out
  Will skip URL http://pypi.python.org/simple/pip when looking for download links for pip
  Getting page http://pypi.python.org/simple/
  Could not fetch URL http://pypi.python.org/simple/: <urlopen error [Errno 113] No route to host>
  Will skip URL http://pypi.python.org/simple/ when looking for download links for pip
  Cannot fetch index base URL http://pypi.python.org/simple/
  URLs to search for versions for pip:
  * http://pypi.python.org/simple/pip/
  Getting page http://pypi.python.org/simple/pip/
  Could not fetch URL http://pypi.python.org/simple/pip/: <urlopen error [Errno 113] No route to host>
  Will skip URL http://pypi.python.org/simple/pip/ when looking for download links for pip
  Could not find any downloads that satisfy the requirement pip
No distributions at all found for pip
Exception information:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 126, in main
    self.run(options, args)
  File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 223, in run
    requirement_set.prepare_files(finder, force_root_egg_info=self.bundle, bundle=self.bundle)
  File "/usr/lib/python2.7/dist-packages/pip/req.py", line 948, in prepare_files
    url = finder.find_requirement(req_to_install, upgrade=self.upgrade)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 152, in find_requirement
    raise DistributionNotFound('No distributions at all found for %s' % req)
DistributionNotFound: No distributions at all found for pip