Tahoe-lafs fails to start with python 2.7 due to missing argparse module

[vrusinov]

Traceback (most recent call last):
  File "/usr/bin/tahoe", line 6, in <module>
    from pkg_resources import load_entry_point
  File "/usr/lib/python2.7/site-packages/pkg_resources.py", line 2707, in <module>
    working_set.require(__requires__)
  File "/usr/lib/python2.7/site-packages/pkg_resources.py", line 686, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/lib/python2.7/site-packages/pkg_resources.py", line 584, in resolve
    raise DistributionNotFound(req)
pkg_resources.DistributionNotFound: argparse>=0.8

But since python 2.7 argparse is included in standard library: ​http://www.python.org/dev/peps/pep-0389/, so my system does not have separate module for argparse.