ValueError: Attempted relative import in non-package

Fixed by changing `from . import iterparse_filter` to `import iterparse_filter` in `setlr/__init__.py`.

Full trace:
```
Traceback (most recent call last):
  File "setlr/__init__.py", line 21, in <module>
    from . import iterparse_filter
ValueError: Attempted relative import in non-package
```