[PATCH] models: Fix HeaderParser import on python 2.4

[Sabuj Pattanayek]

btw, did test that the new imports work in python 2.4, just some other
error in scope somewhere?

> Traceback:
> File "/blue/meilerlab/home/patchwork/lib/python/django/core/handlers/base.py"
> in get_response
>  100.                     response = callback(request,
> *callback_args, **callback_kwargs)
> File "/blue/meilerlab/home/patchwork/apps/patchwork/views/patch.py" in mbox
>  123.     response.write(patch.mbox().as_string(True))
> File "/blue/meilerlab/home/patchwork/apps/patchwork/models.py" in mbox
>  277.         orig_headers = HeaderParser().parsestr(str(self.headers))
>
> Exception Type: NameError at /patch/313/mbox/
> Exception Value: global name 'HeaderParser' is not defined