Fixed #22579 -- Corrected validation for email to reject trailing slash

Thanks to Claude Paroz for the report and patch and Tomasz
Paczkowski for the review.
Fixed #22490 -- removed obsolete ComplexFeed class from syndication_tests

It was previously used in a test that was deprecated and then removed, this cruft can be removed now.
Removed an unused import
Fixed #22531 -- Added tree.Node.__repr__ and tests for the class.

While Node class has a useful `__str__`, its `__repr__` is not that
useful. Added a `__repr__` that makes use of the current `__str__`.
This is especially useful since the more popular `Q` class inherits
`tree.Node`. Also created new tests that cover most of `Node` class
functionality.
Refs #21798 - Modified error number to provide room for future expansion.
Removed --skip-scripts from extras/Makefile.

This is the default now, see https://bitbucket.org/pypa/wheel/commits/c29f82b4cf76df71f8d55b0f825af6063d3b9892
Fixed #18389 -- Fixed the way contribute_to_class is called

Now this method is only called only if the object is an instance.
This allows to have field classes as model class attributes.
Fixed #22622 -- Added ugettext_lazy import in docs
Fixed #22634 -- Changed Session and SessionStore to be easily extendable

Introduced abstract base model for Session, and properties enabling
the developer to override session model class used by session store
and the session store class used by the model.