get_annotation_class_name() not always returning string

[wlupton]

Under python3.8 I get the following crash:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/sphinx/events.py", line 110, in emit
    results.append(listener.handler(self.app, *args))
  File "/usr/local/lib/python3.8/dist-packages/sphinx_autodoc_typehints.py", line 374, in process_docstring
    formatted_annotation = format_annotation(
  File "/usr/local/lib/python3.8/dist-packages/sphinx_autodoc_typehints.py", line 146, in format_annotation
    formatted_args = args_format.format(', '.join(format_annotation(arg, fully_qualified)
  File "/usr/local/lib/python3.8/dist-packages/sphinx_autodoc_typehints.py", line 146, in <genexpr>
    formatted_args = args_format.format(', '.join(format_annotation(arg, fully_qualified)
  File "/usr/local/lib/python3.8/dist-packages/sphinx_autodoc_typehints.py", line 117, in format_annotation
    args = get_annotation_args(annotation, module, class_name)
  File "/usr/local/lib/python3.8/dist-packages/sphinx_autodoc_typehints.py", line 63, in get_annotation_args
    original = getattr(sys.modules[module], class_name)
TypeError: getattr(): attribute name must be string

I didn't get this under python 3.6 so I guess something changed between python 3.6 and 3.8?

It seems that get_annotation_class_name() isn't always returning a string? I added the code shown in the PR (an avoidance measure, not a fix) and now get this:

annotation typing.IO[str] module 'typing' class_name <property object at 0x7fb1c74402c0> <class 'property'>

This is coming (not surprisingly) from this in my python code:

dumpfd: IO[str]

Sorry, but I haven't narrowed it down further (as yet).

[agronholm]

This particular case was not caught by the test suite but the fix proposed here is not the right one in my opinion. I will fix the issue but I will have to turn down this PR, sorry.

[wlupton]

Of course! I didn't think this was the fix; it was the workaround that would (I hope) help to illustrate and narrow down the problem. Thanks.