If you're using virtualenv, and after a system upgrade you get errors like

  File "...", line ...
    from hashlib import md5
  File "/usr/lib/python2.6/hashlib.py", line 63, in __get_builtin_constructor
     import _md5
ImportError: No module named _md5

this means that the copy of the python executable in your virtualenv/bin directory is outdated and you should update it:

$ cp /usr/bin/python2.6 /path/to/venv/bin/python

or, better yet, recreate the virtualenv.