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this means that the copy of the python executable in your virtualenv/bin directory is outdated and you should update it: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