Impossible to install old versions of pip via get-pip

Hi,

I distribute a number of Python packages, and I try to make sure that these packages work with old versions of Python, going back to 2.5. Your move to drop 2.5 broke all my automated tests (thanks!), but what’s worse is that I was unable to install an old version of pip reliably to maintain my ability to test on 2.5. get-pip for version 1.3.1 (which nominally should support 2.5) produces this:

Downloading/unpacking pip
  Running setup.py egg_info for package pip
    warning: no files found matching '*.html' under directory 'docs'
    warning: no previously-included files matching '*.rst' found under directory 'docs/_build'
    no previously-included directories found matching 'docs/_build/_sources'
Installing collected packages: pip
  Running setup.py install for pip
    /var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/lib/python2.5/site-packages/pip/download.py:582: Warning: 'with' will become a reserved keyword in Python 2.6
      File "/var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/lib/python2.5/site-packages/pip/download.py", line 582
        with open(cache_content_type_file) as fp:
                ^
    SyntaxError: invalid syntax
<SNIP>
    SyntaxError: ('future feature unicode_literals is not defined',)

    warning: no files found matching '*.html' under directory 'docs'
    warning: no previously-included files matching '*.rst' found under directory 'docs/_build'
    no previously-included directories found matching 'docs/_build/_sources'
    Installing pip script to /var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/bin
    Installing pip-2.5 script to /var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/bin
Successfully installed pip

Successfully? No, not quite:

+ pip install pytest
Traceback (most recent call last):
  File "/var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/bin/pip", line 9, in <module>
    load_entry_point('pip==1.4', 'console_scripts', 'pip')()
  File "build/bdist.linux-x86_64/egg/pkg_resources.py", line 346, in load_entry_point
  File "build/bdist.linux-x86_64/egg/pkg_resources.py", line 2516, in load_entry_point
  File "build/bdist.linux-x86_64/egg/pkg_resources.py", line 2210, in load
  File "/var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/lib/python2.5/site-packages/pip/__init__.py", line 10, in <module>
    from pip.util import get_installed_distributions, get_prog
  File "/var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/lib/python2.5/site-packages/pip/util.py", line 17, in <module>
    from pip.vendor.distlib import version
  File "/var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/lib/python2.5/site-packages/pip/vendor/distlib/version.py", line 13, in <module>
    from .compat import string_types
  File "/var/lib/jenkins/jobs/pyopencl2/workspace/cl_dev/intel/node/master/py_version/2.5/.env/lib/python2.5/site-packages/pip/vendor/distlib/compat.py", line 276
    return b''
             ^
SyntaxError: invalid syntax

What command will work to reliably install pip across 2.5…3.x the way get-pip.py did until two days ago?

[Ivoz]

One can now use $ python get-pip.py 'pip==N' or $ python get-pip.py 'pip<N'for any version N

[xavfernandez]

#2393 was a PR so I created the corresponding issue #3336 and a new PR: #3337 .