remove internal use of metaclasses

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