Add a few missing types to the stubfiles (#214)

* Add types to stub files.

* Fix black and flake8 errors.

django-stubs/core/mail/backends/smtp.pyi

@@ -0,0 +1,3 @@
+from django.core.mail.backends.base import BaseEmailBackend
+
+class EmailBackend(BaseEmailBackend): ...

django-stubs/db/models/__init__.pyi

@@ -40,6 +40,7 @@ from .fields import (
     DurationField as DurationField,
     BigAutoField as BigAutoField,
     CommaSeparatedIntegerField as CommaSeparatedIntegerField,
+    NOT_PROVIDED as NOT_PROVIDED,
 )
 
 from .fields.related import (

django-stubs/db/models/fields/__init__.pyi

@@ -25,7 +25,7 @@ from django.db.models.expressions import Combinable, Col
 from django.db.models.query_utils import RegisterLookupMixin
 from django.forms import Field as FormField, Widget
 
-from .mixins import NOT_PROVIDED as NOT_PROVIDED
+class NOT_PROVIDED: ...
 
 _Choice = Tuple[Any, Any]
 _ChoiceNamedGroup = Tuple[str, Iterable[_Choice]]
@@ -129,6 +129,7 @@ class Field(RegisterLookupMixin, Generic[_ST, _GT]):
     @property
     def cached_col(self) -> Col: ...
     def value_from_object(self, obj: Model) -> _GT: ...
+    def get_attname(self) -> str: ...
 
 class IntegerField(Field[_ST, _GT]):
     _pyi_private_set_type: Union[float, int, str, Combinable]

django-stubs/forms/__init__.pyi

@@ -2,6 +2,8 @@ from django.core.exceptions import ValidationError as ValidationError
 
 from .forms import Form as Form, BaseForm as BaseForm
 
+from .formsets import BaseFormSet as BaseFormSet, all_valid as all_valid, formset_factory as formset_factory
+
 from .models import (
     ModelForm as ModelForm,
     ModelChoiceField as ModelChoiceField,