Allow using Enum with length=None

Allow using Enum with length=None

Adjusted the :class:`_types.Enum` datatype to accept an argument of
``None`` for the :paramref:`_types.Enum.length` parameter, resulting in a
VARCHAR or other textual type with no length in the resulting DDL. This
allows for new elements of any length to be added to the type after it
exists in the schema.  Pull request courtesy Eugene Toder.

Fixes: #10269
Closes: #10274
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10274
Pull-request-sha: 651afaaea76c1ec868402cdd7106ec8b2de76254

Change-Id: I374ae9e5fa63da21a4cc87e323ce1a54acd6e39b