Impyla

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* **Improvements**
- Unify Python 2 and 3 thrift handling and remove thriftpy2

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* **Improvements**
- Add buffering to hs2-http (442)
- Remove references to 'sasl', always use 'pure-sasl' package

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* **Improvements**
- Switch to using manylinux2010 docker build environment
- Upgrade to thrift-sasl 0.4.3a2

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* **Bug Fixes**
- Add no_utf8strings to thrift compiler option (440)

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* **Improvements**
- Implement simple retry which throws the underlying HttpError if retrying fails
- Ugprade Thift to 0.11.0 for Python 2
- Add build script

* **Bug Fixes**
- Server certs should not be verified if SSL is enabled and ca_cert is not specified
- Added "fetchType" to TFetchResultsReq.
- Fix Thrift compilation with current Impala

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* **Improvements**
- Implement GSSAPI authentication over http transport. (415)
- Vendor thrift classes with Python 3 277 (404)
- Add HTTP code handling to THttpClient (380)
- Disable failing tests - 401 (402)

* **Bug Fixes**
- Fix 418, no 'message', just cast to string (419)
- Fix DATE, VARCHAR handling: 410 (411)