Message 235805 - Python tracker

Posting patch v2:

* Changed readinto() argument descriptions to “a pre-allocated, writable bytes-like buffer”, for both RawIOBase and BufferedIOBase
* Integrated the single-use test_memoryio.BytesIOMixin test class, which tricked me when I did the first patch
* Added tests for BufferedRWPair, BytesIO.readinto() etc methods with non-bytearray() buffers
* Fix _pyio.BufferedReader.readinto/1() for non-bytearray