Update __init__.py
Update __init__.py

Changes
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* Promote `dimod.binary.binary_quadratic_model` to the top-level `dimod` namespace and remove `dimod.binary_quadratic_model.BinaryQuadraticModel` https://github.com/dwavesystems/dimod/pull/847

Fixes
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* Fix `Variables._relabel` in rare cases https://github.com/dwavesystems/dimod/commit/a0791b0045892d383ad37a53da1bf0ff6c0a2bdb
dwavesystems/dimod,dwavesystems/dimod