remove mention to %pylab
Merge pull request #3743 from Carreau/nopylab

DOC : remove mention to %pylab
Fixed a bug that caused rc parameters to not be reset to initial state if an exception happened while setting new rc parameters inside rc_context.
Minor fixes
raise SkipTest if not using Python >= 2.7
Merge pull request #3752 from astrofrog/rc_context_exceptions

BUG : Make sure that initial state gets reset if anything goes wrong in ``rc_context``