Remove ununsed import  error

I can't describe it well in title, so I just show the example.
```python
try:
    import playhouse.postgres_ext as pw_pext
except ImportError:
    pass
```

in this case, pw_pext is unused import.

After `autoflake --in-place --remove-unused-variables --remove-all-unused-imports $file`

```python
try:
    pass
except ImportError:
    pass
```