Thriftrw

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- Fixed bug which caused unrecognized enums to throw exceptions.

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- Add support for string enums in from_primitive.

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- Added support for Python 3.7.
- Removed support for PyPy.
- Removed support for Python 3.3, which has been EOLed as of 2017-09-29.

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- Fixed a bug where errors while parsing certain parts of the Thrift payload
would be ignored.
- Fixed a bug that caused a segfault if a return value was received for a
``void`` function.

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- Further optimizations to the serialization path to reduce unnecessary
python interaction, improving benchmark performance another 4x.
- Optimize deserialization path by doing validation inline at construction
time. This improves deserialization benchmark performance by 2x.

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- Optimized serialization and deserialization performance by removing an
unnecessary intermediate layer.
- Optimized validation at object construction time by skipping the parsing
of nested structs and unions since they would have been validated already
anyway.
- Due to build incompatibility issues, pypy3 support is temporarily being
dropped. The library is unlikely to break, but no guarantees exist.