Fixed Python 2.4 incompatibility introduced in [14218]

sha_constructor was incorrectly used instead of sha_hmac (which only made a
difference under 2.4).

Thanks to Steffan Kaminski for report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14233 bcc190cf-cafb-0310-a4f2-bffc1f526a37