Restructure our python module. A lot.

src/pretix/base/forms.py

@@ -0,0 +1,25 @@
+from django.forms.models import ModelFormMetaclass, BaseModelForm
+from django.utils import six
+from versions.models import Versionable
+
+
+class VersionedBaseModelForm(BaseModelForm):
+    """
+    This is a helperclass to construct VersionedModelForm
+    """
+    def save(self, commit=True):
+        if self.instance.pk is not None and isinstance(self.instance, Versionable):
+            if self.has_changed():
+                self.instance = self.instance.clone()
+        return super().save(commit)
+
+
+class VersionedModelForm(six.with_metaclass(ModelFormMetaclass, VersionedBaseModelForm)):
+    """
+    This is a modified version of Django's ModelForm which differs from ModelForm in
+    only one way: It executes the .clone() method of an object before saving it back to
+    the database, if the model is a sub-class of versions.models.Versionable. You can
+    safely use this as a base class for all your model forms, it will work out correctly
+    with both versioned and non-versioned models.
+    """
+    pass