fix: handle dunder attributes in Wkl.__getattr__ and expose __version__

[rbavery]

Summary

addresses #17

• Fix Wkl.__getattr__ to reject private/dunder attributes (e.g., __version__, __name__) instead of treating them as location chains
• Expose __version__ on the wkls instance using importlib.metadata

Problem

Because wkls replaces its module with a Wkl() instance (sys.modules[__name__] = wkls_instance), accessing any dunder attribute like wkls.__version__ triggers Wkl.__getattr__("__version__").

Previously, this would:

1. Create a location chain ['__version__']
2. Run a SQL query treating __VERSION__ as a country code
3. Return an empty ChainableDataFrame

>>> print(wkls.__version__)
Empty ChainableDataFrame
Columns: [id, country, region, subtype, name_primary, name_en]
Index: []

Solution

1. Add a check in Wkl.__getattr__ to raise AttributeError for attributes starting with _ (matching the existing behavior in ChainableDataFrame.__getattr__)
2. Set __version__ directly on the wkls instance using importlib.metadata.version("wkls"), with a fallback to "0.0.0.dev" for development environments where the package isn't installed

[prantogg]

LGTM. Thanks!