re-arrange mplDeprecation imports

[tacaswell]

in #1917 @NelleV requested I import mplDeprecation directly from cbook. On further investigation I found that none of the current code did that.

This PR changes the import scheme to import directly from cbook everywhere.

[NelleV]

Thanks for doing this work.

You should explicitely import cbook: either use from .cbook import blah or from matplotlib.cbook import blah.
I personnally prefer the first version, but matplotlib's codebase uses the latter (and it is the recommanded way of importing a intra package module).

[tacaswell]

Should lines like

iterable = cbook.iterable
is_string_like = cbook.is_string_like
is_sequence_of_strings = cbook.is_sequence_of_strings

(from axes.py) be changed to from matplotlib.cbook import blah ?

[NelleV]

I think it would be cleaner, but I think this is a question of personal taste so I'll leave it up to you.

[tacaswell]

My inclination is to not change code that works, so I will leave it ;)