Move widget.{get,set}_active to AxisWidget.

The method was previously defined only for RectangleSelectors (#3375).

Also, an redundant line was removed (RectangleSelectors are already set
active on __init__ by the superclass).
Update background on widget.set_active(True).

Because selectors must update their background on set_active(True),
widget.active was made a property for backwards compatibility.
Updated whats_new.rst.

Because _Selector is a private class there are no references to the
actual getter and setter in the rst file.
Minor fixes according to Github discussion.
_SelectorWidget overrides the active property.

... so it should dynamically retrieve the set_active method.
Better repr for Bboxes.
Actually eval'able __repr__ for Bboxes.
__str__ & __format__ for Bboxes, and tests.
pep8ify: examples/animation

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
pep8ify: examples/api

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
pep8ify: examples/axes_grid

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
add parens for the sake of clarity
fix awkward line break induced by autopep8
minor cleanup in collections_demo.py
fix some pep8 issues overseen by autopep8
test the pep8 conformance of the mpl examples
fix some pep8 issues overseen by autopep8
pep8: there are no exp. bad files for the examples
fix too many blank lines
refactor pep8 conformance test