Fix sqlalchemy.ddl.DDL.execute_if dialect type (#181)

[philpep]

dialect should be a string a list/tuple/set of strings. Not a Dialect object.

https://github.com/sqlalchemy/sqlalchemy/blob/rel_1_4_18/lib/sqlalchemy/sql/ddl.py#L158

Closes #181

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jace]

Wow, this has been stuck a while.