add user_module_prefix param for render_python_code

add user_module_prefix param for render_python_code

Fixed issue where :func:`.autogenerate.render_python_code` function did not
provide a default value for the ``user_module_prefix`` variable, leading to
``NoneType`` errors when autogenerate structures included user-defined
types. Added new parameter
:paramref:`.autogenerate.render_python_code.user_module_prefix` to allow
this to be set as well as to default to ``None``. Pull request courtesy
tangkikodo.

Fixes #1235
Closes: #1233
Pull-request: https://github.com/sqlalchemy/alembic/pull/1233
Pull-request-sha: 6fa3bc1ce4f2130bf7a28282a84e3bb9a3223304

Change-Id: Ic21eec87f373a9518513a6a308f16e5d2b5ebf81