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"InvalidRequirement: Invalid requirement, parse error" error after updating a python package

After updating a package (IPython in my case) using pip install -U ipython running any Python script that uses entry points fails with this error:

Traceback (most recent call last):
  File "/home/adrian/dev/indico/env/bin/indico", line 5, in <module>
    from pkg_resources import load_entry_point
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/pkg_resources/_vendor/packaging/requirements.py", line 94, in __init__
    requirement_string[e.loc:e.loc + 8], requirement_string))
pkg_resources._vendor.packaging.requirements.InvalidRequirement: Invalid requirement, parse error at "'< 2.0'"
Nothing else changed, I did not update any other libraries.
This is caused by an issue in setuptools==20.2.1 which is pulled in by IPython (setuptools>..), so a pip install -U updated it.
Until a fixed version is released or the broken version is pulled from PyPI there is a simple workaround (but note that it will break again if something updates setuptools):
pip install -U pip
pip uninstall setuptools
pip install 'setuptools<20.2'
The pip update is needed since older versions of pip will not work without setuptools being installed
See these IRC logs and BitBucket issue for details:
http://chat-logs.dcpython.org/day/pypa/2016-02-25
https://bitbucket.org/pypa/setuptools/issues/502/packaging-164-does-not-allow-whitepace
        
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