'Produce a new type object that will utilize the given
type when applied to the dialect of the given name.
e.g.::
from sqlalchemy.types import String
from sqlalchemy.dialects import mysql
s = String()
s = s.with_variant(mysql.VARCHAR(collation=\'foo\'), \'mysql\')
The construction of :meth:`.TypeEngine.with_variant` is always
from the "fallback" type to that which is dialect specific.
The returned type is an instance of :class:`.Variant`, which
itself provides a :meth:`~sqlalchemy.types.Variant.with_variant`
that can be called repeatedly.
:param type_: a :class:`.TypeEngine` that will be selected
as a variant from the originating type, when a dialect
of the given name is in use.
:param dialect_name: base name of the dialect which uses
this type. (i.e. ``\'postgresql\'``, ``\'mysql\'``, etc.)
.. versionadded:: 0.7.2'

def with_variant(self, type_, dialect_name):

    return Variant(self, {dialect_name: type_})
