breaking change : "form" -> "parler_base_form_class"

[weber-s]

It seems that commit 5f9c336 add a breaking change in the base class form of modeladmin/snippet form.

It used to be self.form, and it's now self.parler_base_form_class.
Moreover, this attribute is not documented and may clash with already defined base_form_class, i.e. this edit_handler will not use the custom form and will raise an error:

class CutstomForm(WagtailAdminModelForm):
    variableA = forms.CharField()


class CustomAdmin(ParlerSnippetAdminMixin, SnippetViewSet):
    model = Custom
    edit_handler = ObjectList(
       children=[Field("existing_field"),Field("variableA")],
       base_form_class=CustomForm
    )