See upstream for fix: https://savannah.nongnu.org/bugs/index.php?19998 And launchpad: https://bugs.launchpad.net/ubuntu/feisty/+source/duplicity/+bug/88617
Another solution will be a version bump to 0.4.3-rc10, which is the lastes version available (released 14 July) and has the patches for making this python-2.5 compatible. Come on, guys, aplly these patches or bump to a newer version. This is an important bug, is one of the few who blocks python-2.5 unmasking.
Also note that 0.4.3-RC11 needs dev-python/py-gnupg to run too!
please see http://bugs.gentoo.org/show_bug.cgi?id=148333#c9
Fixed in 0.4.2-r2