Fixed #21560 -- missing 'slug' field in example code

I updated the documentation, that the modelform now includes the 'slug' field.

This is a partial undo of 1b142ef5.

Made flake8 ignore the .git directory.

Trigger AttributeError in ValidationError.message_dict when error_dict is missing.

Renamed first argument of class method to cls

I've chosen a quick deprecation path, as Geo Sitemaps themselves
are deprecated from some time now.

The goal of this change is twofold; firstly, matching the behavior of Django 1.6
and secondly, an AttributeError is more informative than an obscure ValueError
about mismatching sequence lengths.

Refs #20867.

With Python 2.7 and up, named parameter keys are not limited to
bytestrings any longer. This mainly reverts 3bd384aa.

Thanks trac username zanuxzan for the report and original patch.

This was a regression in Django 1.6 that was only partially
restored in 839940f2.
Thanks Jonas Haag for the report.

Thanks dibrovsd at gmail.com for the report and Simon Charette
for the review.

Fixed #21572 -- Added unit test for django.utils.text.normalize_newlines...

Thanks to @dmyerscoug for the report and original patch
and to @alasdairnicol for the added tests.

This is for general consistency in the GeoDjango DB backends.
Thanks Claude for the fix.

Refs #19678.

Refs #21010.

Thanks to Loïc for the patch.

Fixed TypeError when rendering ModelState with multiple bases.

Thanks Julien for the report and Claude for the fix.

Thanks yourcelf for the report.