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Issue/948 (PyCQA#949)

* Implement fix for stream processing on Python 2.7

* Bump version for immediate distribution

* Add change-set and release date to changelog

Issue/942 (PyCQA#946)

* Add test cases for .pyi extension support, in particular from file based loading

* Implement native support for pyi files

* Bump version to prepare for immediate deploy

Hot-fix Release 4.3.18

Release version 4.3.17

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