Debian – pip install gives “TypeError: ‘int’ object is not iterable ”

In Debian Stretch, when I try to install the python package python-constraint via

pip install python-constraint

I get the following error;

Exception:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 122, in main
    status = self.run(options, args)
  File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 290, 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 1178, in prepare_files
    url = finder.find_requirement(req_to_install, upgrade=self.upgrade)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 292, in find_requirement
    elif is_prerelease(version) and not (self.allow_all_prereleases or req.prereleases):
  File "/usr/lib/python2.7/dist-packages/pip/util.py", line 739, in is_prerelease
    return any([any([y in set(["a", "b", "c", "rc", "dev"]) for y in x]) for x in parsed])
TypeError: 'int' object is not iterable

Storing debug log for failure in /home/von/.pip/pip.log

In Debian Jessie the same command is sucessful.

Where is the problem? How to solve it?

$python --version
Python 2.7.9

$pip --version
pip 1.5.6 from /usr/lib/python2.7/dist-packages (python 2.7)