analyze: explicitly use gdbm as dbm

and use six.moves for python3 compatibility.

Previously we were using "anydbm" which selects dbhash, gdbm or dbm, but
opening the db with the f flag that's only valid for gdm, so de-facto we
were supporting only gdbm.

zodbtools/zodbanalyze.py

@@ -9,7 +9,7 @@ from __future__ import print_function
 import sys
 import os
 import getopt
-import anydbm as dbm
+from six.moves import dbm_gnu as dbm
 import tempfile
 import shutil
 from ZODB.FileStorage import FileIterator, packed_version