Fix "dictionary changed size during iteration error"

[DrDaveD]

I got the following error with osg-notify:

$ osg-notify --type production --no-sign --message /etc/motd --subject 'OASIS upgrade' --recipients "dwd@fnal.gov" --oim-recipients resources --oim-contact-type administrative --dry-run
Traceback (most recent call last):
  File "/nashome/d/dwd/work/topology/bin/osg-notify", line 250, in <module>
    main()
  File "/nashome/d/dwd/work/topology/bin/osg-notify", line 192, in main
    results = topology_utils.filter_contacts(args, results)
  File "/nashome/d/dwd/work/topology/src/topology_utils.py", line 345, in filter_contacts
    for name in results.keys():
RuntimeError: dictionary changed size during iteration

This PR applies a hint from stack overflow.

[brianhlin]

LGTM! Thanks for the fix