Rename _getsize to __len__

[google-labs-jules]

Renamed _getsize to __len__ and removed the alias in BitVector/BitVector.py as requested. Checked for references of _getsize in the rest of the codebase (there were none). Tests passed successfully using uv run pytest.

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PR created automatically by Jules for task 2933394259028951356 started by @schwehr

[google-labs-jules]

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