remove internal use of metaclasses

All but one metaclass used internally can now
be replaced using __init_subclass__().  Within this
patch we remove:

* events._EventMeta
* sql.visitors.TraversibleType
* sql.visitors.InternalTraversibleType
* testing.fixtures.FindFixture
* testing.fixtures.FindFixtureDeclarative
* langhelpers.EnsureKWArgType
* sql.functions._GenericMeta
* sql.type_api.VisitableCheckKWArg (was a mixture of TraversibleType
  and EnsureKWArgType)

The remaining internal class is MetaOptions used by the
sql.Options object which is in turn currently mostly for
ORM internal use, as this type implements class level overrides
for the ``+`` operator.

For declarative, removing DeclarativeMeta in place of
an `__init_subclass__()` class would not be fully feasible as
it would break backwards compatibility with applications that
refer to this class explicitly, but also DeclarativeMeta intercepts
class-level attribute set and delete operations which is a widely
used pattern.   An option for declarative base to use
`__init_subclass__()` should be provided but this is out of
scope for this particular change.

Change-Id: I8aa898c7ab59d887739037d34b1cbab36521ab78
References: #6810

1 files changed, 4 insertions, 4 deletions

diff --git a/lib/sqlalchemy/ext/mutable.py b/lib/sqlalchemy/ext/mutable.py
index a2e1a3826..d75cf667b 100644
--- a/lib/sqlalchemy/ext/mutable.py
+++ b/lib/sqlalchemy/ext/mutable.py
@@ -269,10 +269,10 @@ and to also route attribute set events via ``__setattr__`` to the
         def __ne__(self, other):
             return not self.__eq__(other)
 
-The :class:`.MutableComposite` class uses a Python metaclass to automatically
-establish listeners for any usage of :func:`_orm.composite` that specifies our
-``Point`` type. Below, when ``Point`` is mapped to the ``Vertex`` class,
-listeners are established which will route change events from ``Point``
+The :class:`.MutableComposite` class makes use of class mapping events to
+automatically establish listeners for any usage of :func:`_orm.composite` that
+specifies our ``Point`` type. Below, when ``Point`` is mapped to the ``Vertex``
+class, listeners are established which will route change events from ``Point``
 objects to each of the ``Vertex.start`` and ``Vertex.end`` attributes::
 
     from sqlalchemy.orm import composite, mapper