[examples] fix pep8 errors E26*

and minor modification of the output of autopep8 where appropriate
[examples] switch to print function everywhere + fix E211

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
Change order of includes for Ubuntu
adding legend handler for PolyCollection
adding labels to stackplot
removing comments and unnecessary line changes
added test image and entry in whats_new
moved stackplot change in 1.5 section of whats_new
Merge pull request #3303 from fcolas/master

ENH : Adding legend handler to PolyCollection and labels to stackplot
Make png.h the absolute first thing
readability improvement in usetex_demo
Merge pull request #3668 from thisch/pep8fix26

[examples] pep8 fix E26*
Merge pull request #3684 from mdboom/fix-order-of-includes

Build failure on Launchpad