'PostgreSQL' object has no attribute '_conn'

**Describe the bug**
An exception occurs when using data-diff with Postgres

```
data_diff/__main__.py", line 485, in _data_diff
    if db1 == db2:
       ^^^^^^^^^^
  File "<attrs generated eq data_diff.databases.postgresql.PostgreSQL>", line 13, in __eq__
AttributeError: 'PostgreSQL' object has no attribute '_conn'
```

**Describe the environment**

data-diff 0.9.0
