Don't pop from currently iterated dict

[nphilipp]

Avoids: "RuntimeError: dictionary changed size during iteration"

Context: https://fedora.softwarefactory-project.io/zuul/build/c5d36ac22ef741b4b2e5abb2f7a8cd29

Traceback (most recent call last):
  File "/usr/bin/ansible-review", line 11, in <module>
    load_entry_point('ansible-review==0.13.9', 'console_scripts', 'ansible-review')()
  File "/usr/lib/python3.7/site-packages/ansiblereview/__main__.py", line 99, in main
    errors = errors + candidate.review(options, lines)
  File "/usr/lib/python3.7/site-packages/ansiblereview/__init__.py", line 76, in review
    return utils.review(self, settings, lines)
  File "/usr/lib/python3.7/site-packages/ansiblereview/utils/__init__.py", line 120, in review
    result = standard.check(candidate, settings)
  File "/usr/lib/python3.7/site-packages/ansiblereview/groupvars.py", line 89, in same_variable_defined_in_competing_groups
    remove_inherited_and_overridden_group_vars(group, inv)
  File "/usr/lib/python3.7/site-packages/ansiblereview/groupvars.py", line 53, in remove_inherited_and_overridden_group_vars
    remove_inherited_and_overridden_vars(_vars[group], ancestor, inventory)
  File "/usr/lib/python3.7/site-packages/ansiblereview/groupvars.py", line 41, in remove_inherited_and_overridden_vars
    for (k, v) in vars.items():
RuntimeError: dictionary changed size during iteration