Features
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* General Performance Improvements for Throughput (PYTHON-283)
* Improve synchronous request performance with Timers (PYTHON-108)
* Enable C Extensions for PyPy Runtime (PYTHON-357)
* Refactor SerDes functionality for pluggable interface (PYTHON-313)
* Cython SerDes Extension (PYTHON-377)
* Accept iterators/generators for execute_concurrent() (PYTHON-123)
* cythonize existing modules (PYTHON-342)
* Pure Python murmur3 implementation (PYTHON-363)
* Make driver tolerant of inconsistent metadata (PYTHON-370)
Bug Fixes
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* Drop Events out-of-order Cause KeyError on Processing (PYTHON-358)
* DowngradingConsistencyRetryPolicy doesn't check response count on write timeouts (PYTHON-338)
* Blocking connect does not use connect_timeout (PYTHON-381)
* Properly protect partition key in CQL export (PYTHON-375)
* Trigger error callbacks on timeout (PYTHON-294)