allow selecting the backend by setting the environment variable MPL_BACKEND
added documentation on backend selection methods
minor fix for the documentation on backend selection methods (either of the methods -> any of the methods)
DOC : edits to copy of usage_faq.rst

Made how to set backend an ordered list
copy all array_view members in copy constructor
Use XINCREF in copy constructor and subview constructor, since arr may be NULL
Added a warning about global use of MPLBACKEND or use of -dbackend
remove unused parameter 'contiguous'
Merge pull request #3811 from jbmohler/array_view_ctor

copy all array_view members in copy constructor
Added mplDeprecation warning when the -d argument is used. Minor improvements to the backend selection documentation.
Make array_view::operator= non-const
Merge pull request #3710 from FlorianRhiem/master

allow selecting the backend by setting the environment variable MPLBACKEND
[examples] use np.radians/np.degrees where appropriate
fix typo

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
Merge pull request #3814 from thisch/radians

[examples] use np.radians/np.degrees where appropriate
Merge pull request #3813 from mdboom/non-const-operator=

Make array_view::operator= non-const
Use StringIO from io in python3
No need to use xrange for a short loop

fixes python3
marker_reference python 3 compatible
boxplot_demo2 python3