`AttributeError` using `model_form` on models using Joined Table Inheritance

[mfisher87]

When I'm using Joined Table Inheritance by specifying __mapper_args__ in my model, I get the following error calling model_form() on my model:

AttributeError: Neither 'Label' object nor 'Comparator' object has an attribute 'nullable'

I can get around this by making the following change to model_fields():

      field_args = field_args or {}
      properties = []

      for prop in mapper.attrs.values():
+         if getattr(prop, "_is_polymorphic_discriminator", False):
+             continue
          if getattr(prop, "columns", None):
              if exclude_fk and prop.columns[0].foreign_keys:
                  continue

I don't have deep enough knowledge of sqlalchemy to know if there is a better field I should be checking. If this looks right, I'm happy to open a PR.