cannot import name MAXREPEAT

When I upgraded from Xubuntu 12.10 to 13.04 today, all my existing Python virtualenvs broke! Fortunately, they're just virtualenvs and easy to replace (that's kind of the point). But don't panic if you start seeing these.

$ ipython
Traceback (most recent call last):
  File "/home/catherine/ve/e2/bin/ipython", line 5, in 
    from pkg_resources import load_entry_point
  File "build/bdist.linux-i686/egg/pkg_resources.py", line 16, in 
  File "/home/catherine/ve/e2/lib/python2.7/re.py", line 105, in 
    import sre_compile
  File "/home/catherine/ve/e2/lib/python2.7/sre_compile.py", line 14, in 
    import sre_parse
  File "/home/catherine/ve/e2/lib/python2.7/sre_parse.py", line 17, in 
    from sre_constants import *
  File "/home/catherine/ve/e2/lib/python2.7/sre_constants.py", line 18, in 
    from _sre import MAXREPEAT
ImportError: cannot import name MAXREPEAT

Apparently Python 2.7.4 introduces _sre.MAXREPEAT. Here it is in my (new) system Python, 2.7.4:

Python 2.7.4 (default, Apr 19 2013, 18:28:01) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import _sre
>>> _sre.MAXREPEAT
4294967295L 

... but the virtualenvs I created before the upgrade still use Python 2.7.3

Python 2.7.3 (default, Sep 26 2012, 21:51:14) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import _sre
>>> _sre.MAXREPEAT
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'module' object has no attribute 'MAXREPEAT'

If I were a deeper hacker I'd try to figure out why code running within my old virtualenvs is trying to access a 2.7.4-only attribute, or what's the most efficient way to recover my old virtualenvs. But I'll settle for recognizing the problem and spinning up new virtualenvs instead. That fixes the problem.

I know... I could have avoided this problem by using Python 3! I've got code that depends on fabric, though, which still isn't available for 3.