in python2 progress's .update is not getting outputs "live"

[yarikoptic]

Possibly related #583, #145 Initially noted while adding progress reporting in datalad: https://github.com/datalad/datalad/pull/2876 where I have not observed it properly (python2) and @kyleam said that he sees them nicely (but he is using python3). Then reconfirmed with the minimal example in #798 - in python 3.6.5 I am getting update calls as underlying git process updates stderr, while in python 2.7.14+ they are just getting all flushed at once probably whenever git process already has done all of them. So somewhere a buffer of some kind prevents "live" retrieval of the stderr from git call.