[ZODB-Dev] No longer using the ZODB3-3_1-branch

[Jeremy Hylton]

We have decided to do any further development of ZODB 3.1 on the
Zope-2_6-branch.  This change will prevent unintentional differences
between ZODB and Zope releases and will minimize the amount of porting
we have to do between branches.

If you have a ZODB3 checkout on the old branch, please update it to the
Zope-2_6-branch.

We expect to do the same with the ZODB3-3_2-branch soon.

Jeremy