refactor: AttributeError handling for sphinx.util.inspect.get* is not needed

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• Internally, getall(), getannotations(), getmro() and getslots() in sphinx.util.inspect use safe_getattr() with the default keyword. Therefore it never raises AttributeError even if the subject raises an error on accessing __mro__ attribute.
• This fixes the wrong their usage.

[tk0miya]

I confirmed with sphinx.util.inspect.getannotations() suppresses errors on accessing its attribute with following code:

>>> class Foo:
...     @property
...     def __annotations__(self):
...         raise
...
>>>
>>> foo = Foo()
>>> foo.__annotations__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __annotations__
RuntimeError: No active exception to reraise
>>> from sphinx.util import inspect
>>> inspect.getannotations(foo)
{}