Fix bind method in some classes. It's a @property

[svalentin]

class Executable has this method as @property. Some subclasses didn't have it as such. mypy will start checking this in v0.990

[svalentin]

It's defined here https://github.com/dropbox/sqlalchemy-stubs/blob/master/sqlalchemy-stubs/sql/base.pyi#L48 And there are other sub classes that have it as @property. I am pretty sure it should be a @property