Bump to 1.7.23 dev.
Clean up PyFlakes warnings in 1.7.

In order to keep things clean, I've gone through the PyFlakes warnings
found in 1.7 and fixed them up. These fixes match the fixes done in
2.0, and shouldn't be too painful to merge.
Fix the MySQL docs for max_allowed_packet.

The old docs were not only styled wrong, but were also only showing how
to set the per-session max allowed packet size. This updates the docs to
show where to set this permanently, and to also discuss setting it just
for the session (switching the example to using mysqldump).
Merge branch 'release-1.7.x' into release-2.0.x