unmapped_dataclass

unmapped_dataclass

Reworked the handling of classes which extend from MappedAsDataclass but
are not themselves mapped, i.e. the declarative base as well as any
mixins or abstract classes.  These classes as before are turned into
real dataclasses, however a scan now takes place across the mapped
elements such as mapped_column(), relationship(), etc. so that we may
also take into account dataclasses.field-specific parameters like
init=False, repr, etc.  The main use case for this is so that mixin
dataclasses may make use of "default" in fields while not being rejected
by the dataclasses constructor.  The generated classes are more
functional as dataclasses in a standalone fashion as well, even though
this is not their intended use.  As a standalone dataclass, the one
feature that does not work is a field that has
a default with init=False, because we still need to have a
mapped_column() or similar present at the class level for the class
to work as a superclass.

The change also addes the :func:`_orm.unmapped_dataclass` decorator
function, which may be used
to create unmapped superclasses in a mapped hierarchy that is using the
:func:`_orm.mapped_dataclass` decorator to create mapped dataclasses.
Previously there was no way to use unmapped dataclass mixins with
the decorator approach.

Finally, the warning added in 2.0 for :ticket:`9350` is turned into
an error as mentioned for 2.1, since we're deep into dataclass hierarchy
changes here.

Fixes: #12854
Change-Id: I11cd8c628d49e9ff1bdbda8a09f4112b40d84be7