Refs #24487 -- Added upgrade tips about removal of SortedDict.

As :class:`~collections.OrderedDict` was added to the standard library in

Python 2.7, ``SortedDict`` is no longer needed and has been deprecated.

The two additional, deprecated methods provided by ``SortedDict`` (``insert()``

and ``value_for_index()``) have been removed. If you relied on these methods to

alter structures like form fields, you should now treat these ``OrderedDict``\s

as immutable objects and override them to change their content.

For example, you might want to override ``MyFormClass.base_fields`` (although

this attribute isn't considered a public API) to change the ordering of fields

for all ``MyFormClass`` instances; or similarly, you could override

``self.fields`` from inside ``MyFormClass.__init__()``, to change the fields

for a particular form instance. For example (from Django itself)::

    PasswordChangeForm.base_fields = OrderedDict(

        (k, PasswordChangeForm.base_fields[k])

        for k in ['old_password', 'new_password1', 'new_password2']

    )

Custom SQL location for models package

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