Improve type of BaseForms.files (#350)

This is valid Django, which failed to pass with the old type-defintion:

    class MyForm(Form):
      myparam = MultiFileField(...)

      self.clean(self, *args, **kwargs):
        self.files.getlist('myparam', [])

django-stubs/forms/forms.pyi

@@ -1,11 +1,13 @@
 from typing import Any, Dict, Iterator, List, Mapping, Optional, Sequence, Type, Union
 
 from django.core.exceptions import ValidationError as ValidationError
+from django.core.files import uploadedfile
 from django.forms.boundfield import BoundField
 from django.forms.fields import Field
 from django.forms.renderers import BaseRenderer
 from django.forms.utils import ErrorDict, ErrorList
 from django.forms.widgets import Media, MediaDefiningClass
+from django.utils.datastructures import MultiValueDict
 from django.utils.safestring import SafeText
 
 class DeclarativeFieldsMetaclass(MediaDefiningClass): ...
@@ -18,7 +20,7 @@ class BaseForm:
     use_required_attribute: bool = ...
     is_bound: bool = ...
     data: Dict[str, Any] = ...
-    files: Optional[Dict[str, Any]] = ...
+    files: MultiValueDict[str, uploadedfile.UploadedFile] = ...
     auto_id: str = ...
     initial: Dict[str, Any] = ...
     error_class: Type[ErrorList] = ...