Typo
Changed the 'ZODB' extra to require only the real 'ZODB' package.

Previously, the extra required the 'ZODB3' metapackage:  depending on the
version used, the metapackage could pull in stale versions of **this**
package and 'persistent'.
Prep 4.0.6 release.
Merge branch 'master' of github.com:zopefoundation/BTrees
Changelog for setup.py version fix.