fix forgotten cStringIO replacement when replacing 2to3 with six

[davidovitch]

...x

Error introduced with commit f4adec7 (Use six instead of 2to3). For Python2 cStringIO was still being used, but the import statement was removed. Both Python2 and 3 should use io.StringIO, which is the same as elsewhere in the code.

[mrterry]

Are io.StringIO and cStringIO.StringIO the same on python 2? Why not use six.moves.cStringIO?

[mdboom]

Yeah -- I think it needs to be six.moves.cStringIO -- it needs to be for bytes on Python 2 and unicode on Python 3.

[davidovitch]

Thanks for reviewing. I'll close this one and make a new PR but not against the master, but a bug-tree. If I understand it correctly that is the preferred way of working, right?

[efiring]

@davidovitch I'm not sure I understand your question. The procedure is to make a branch in your clone, starting from current master; make the change in that branch; push that branch to your clone on github; and make the pull request for master to pull your branch. That is usually what is meant by a "PR against master": the github matplotlib master is asked to pull in a branch with a different name from your github repo, where you made that branch relative to current matplotlib master.

[davidovitch]

@efiring thanks for clarifying. My previous question was not formulated correctly, but the work flow you describe is what I had in mind. If I understand it correctly, this PR violated the outlined work flow. So that's why I closed it, implemented the comments, and re-opened it in PR #2393 using the correct work flow. Apologies for these beginner mistakes.