fix dictionary changed size during iteration

[klorenz]

Recently had the issue:

Exception occurred:
  File "/path/to/venv/lib/python3.8/site-packages/sphinxcontrib/domaintools/__init__.py", line 109, in clear_doc
    for key, (fn, _) in self.data['objects'].items():
RuntimeError: dictionary changed size during iteration

[imphil]

I'm seeing this problem regularly, and the fix LGTM. Can we get it merged? @klorenz @tk0miya?

[eric-wieser]

Though #10 also has a fix for the same problem.