Xxhash

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- Upgrade xxHash to v0.7.1

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- Wheels are now built automatically
- Split CFFI variant into a separate package `ifduyue/python-xxhash-cffi <https://github.com/ifduyue/python-xxhash-cffi>`_

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- Add oneshot functions xxh{32,64}_{,int,hex}digest

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- Allow input larger than 2GB
- Release the GIL on sufficiently large input
- Drop support for Python 3.2

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- Free state actively, instead of delegating it to ffi.gc

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- Fixed copy() segfault
- Added CFFI variant