[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
Remove the check on path length over 18980 in Cairo backend
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
Merge pull request #3702 from agardelein/master

MNT : Remove the check on path length over 18980 in Cairo backend