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Move requirements to motor>=2.0.
Remove ``count`` method from ``MotorQuerySetCursor``.
Add session support to ``MotorQuerySet`` and ``Document``.
Add ``create_collection`` method to ``Document``.
Fix ``__aiter__`` of ``MotorQuerySetCursor`` for python 3.7.
Deprecate ``count`` method of ``MotorQuerySet``.
Deprecate ``create`` method of ``Document``.