Can reproduce this:

>>> ExampleCls.__dict__
mappingproxy({'__module__': '__main__', 'iter_cls': <classmethod(<function ExampleCls.iter_cls at 0x7f1742669d80>)>, '__dict__': <attribute '__dict__' of 'ExampleCls' objects>, '__weakref__': <attribute '__weakref__' of 'ExampleCls' objects>, '__doc__': None})
>>> ExampleCls.iter_cls()
{}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in iter_cls
RuntimeError: dictionary changed size during iteration
>>> ExampleCls.__dict__
mappingproxy({'__module__': '__main__', 'iter_cls': <classmethod(<function ExampleCls.iter_cls at 0x7f1742669d80>)>, '__dict__': <attribute '__dict__' of 'ExampleCls' objects>, '__weakref__': <attribute '__weakref__' of 'ExampleCls' objects>, '__doc__': None, '__annotations__': {}})

The descriptor for type.__annotations__ writes a new attribute into the class's dict if it doesn't exist yet. This was done in bpo-43901 / GH-25263.

I don't see a way to fix this bug while preserving the behavior bpo-43901 was aiming for.