copy dictionary keys before deleting them
copy dictionary keys before deleting them

Under Python3 it appears that you can't modify a list of keys while iterating
over them. https://docs.python.org/3/library/stdtypes.html#dict-views

Under Python3 you'll see an error like this unless a copy of the keys is
created:

```
python3 appdeps.py --wait-secs 60 --port-wait localhost:8080
Check 1
localhost:8080 is available.
Traceback (most recent call last):
  File "appdeps.py", line 90, in <module>
    for check_iter in iters:
RuntimeError: dictionary changed size during iteration
```

gwu-libraries/appdeps