duplicity crashes when PYTHONOPTIMIZE is set

[az]

this is a copy of debian #659532, which lives here: bugs.debian.org/659532
original report:
With the environment variable PYTHONOPTIMIZE set, duplicity doesn't
work. Backups can be created, but it is unable to understand (in
particular check/increment) its own backups, claiming that either
signatures or data is missing. This affects several versions of
duplicity 0.6, and concerns even the simplest of backup cases: local to
local w/o encryption.

I spent a _lot_ of time trying to find the problem, so I didn't debug
this any further. However it seems rdiff also has issues with
PYTHONOPTIMIZE (see https://bugs.launchpad.net/ubuntu/+source/rdiff-backup/+bug/658413),
perhaps there is similar or even shared code.

Either way, it would be nice to document the clash with PYTHONOPTIMIZE
in the man page, or add a check to the code that informs the user that
it has to be unset.