scripts/netspace: Fix so it runs

Currently that script does not run because of import error:

    $ python src/ZODB/scripts/netspace.py
    Traceback (most recent call last):
      File "src/ZODB/scripts/netspace.py", line 13, in <module>
        from ZODB.referencesf import referencesf
    ImportError: No module named referencesf

and that ZODB.referencesf module was moved to ZODB.serialize long ago in

    ec014812    (Move referencesf to ZODB.serialize.)
    ae7e113e    (Ack!  Deleted the wrong file in the last checkin.)
    355ff4ac    (One more try.  Commit the version with referencesf.)

only the conversion missed the netspace.py script.

Fix it.

src/ZODB/scripts/netspace.py

@@ -10,7 +10,7 @@ from __future__ import print_function
 import ZODB
 from ZODB.FileStorage import FileStorage
 from ZODB.utils import U64, get_pickle_metadata
-from ZODB.referencesf import referencesf
+from ZODB.serialize import referencesf
 from six.moves import filter
 
 def find_paths(root, maxdist):