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

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 dropbox#181

Author: philpep

Diff for: sqlalchemy-stubs/sql/ddl.pyi

@@ -33,7 +33,7 @@ class DDLElement(Executable, _DDLCompiles):
     def execute_at(self, event_name: str, target: SchemaItem) -> None: ...
     def against(self: _DDLE, target: SchemaItem) -> _DDLE: ...
     state: Any = ...
-    def execute_if(self: _DDLE, dialect: Optional[Dialect] = ..., callable_: Optional[_DDLCallable] = ...,
+    def execute_if(self: _DDLE, dialect: Optional[Union[str, Sequence[str]]] = ..., callable_: Optional[_DDLCallable] = ...,
                    state: Optional[Any] = ...) -> _DDLE: ...
     def __call__(self, target: Optional[Union[Table, MetaData]], bind: Connection, **kw: Any) -> Optional[ResultProxy]: ...