`has_fields` leads to `AttributeError: __mro__` when the argument provided is a `typing.Union`

[smarie]

from typing import Union
from pyfields import has_fields

has_fields(Union[int, float])

Leads to a non-user-friendly error message:

Traceback (most recent call last):
  ...
  File "<ipython-input-4-0e8bf0e2111b>", line 1, in <module>
    has_fields(Union[int, float])
  File "\site-packages\pyfields\helpers.py", line 146, in has_fields
    return any(yield_fields(cls, include_inherited=include_inherited))
  File "\site-packages\pyfields\helpers.py", line 71, in yield_fields
    where_cls = reversed(getmro(cls)) if ancestors_first else getmro(cls)
  File "C:\Python38\lib\inspect.py", line 489, in getmro
    return cls.__mro__
  File "C:\Python38\lib\typing.py", line 760, in __getattr__
    raise AttributeError(attr)
AttributeError: __mro__

We should rather return None or raise a proper ValueError indicating that cls is not an actual class.