Support UUID type in the sqlalchemy dialect

 We are facing an issue with the cockroachdb sqlalchemy dialect. We use UUID types in our application but the dialect doesn't support it. Our current solution is to patch the dialect.py and add the UUID type imported from sqlalchemy to the _type_map like so:
```
from sqlalchemy.dialects.postgresql import UUID

_type_map = dict(
    ...
    uuid=UUID,
)
```