It occured to me last night that it could be checked using a regular
expression, and indeed the locale module already has a regular
expression that matches percent codes.  I've uploaded a patch that uses
this regex to fix this issue.  I've removed 2.6 and 3.0 as this change
could break existing code that is misusing format.

I added georg.brandl to the nosy list since svn blame shows him as the
author of the code being modified.