DOC: clean up of demo_annotation_box.py

[afvincent]

Clean up of pylab_examples/demo_annotation_box.py:

• moves all imports to the head of the script;
• reorganizes and adds comments to make the different steps more obvious;
• removes the final plt.draw call;
• loads Grace Hopper's portrait with plt.imread instead of _png.read_png, which (at least) avoids an additional import (besides, is the _png supposed to be exposed in a pylab example?).

[afvincent]

It looks to me that the Travis pep8 failure is not related the current PR (the incriminated file is test_backend_svg.py…), but tell me if I'm wrong and I will re-check the pep8-ness of the file I've modified.

[WeatherGod]

yeah, the fix for that is in master now, so don't fret it.

[tacaswell]

backported to v2.x as 6b3622a