Attached is an updated patch that 
 - removes the code duplication in _pyio.BufferedIOBase
 - adds an internal _readinto helper method to _pyio.BufferedReader that makes the implementation similar to io.BufferedReader.
 - implements _pyio.BuffereadReader.{readinto,readinto1} in terms of the new helper method and, as a side effect, also increases their performance.


Performance of the _pyio implementation on my system is:

pre-patch:
readinto:  5.130e+00 seconds
readinto1 not available

post-patch:
readinto:  2.039e+00 seconds
readinto1: 1.995e+00 seconds