Added missing warnings import

There is a warnings call around line 732, but as warnings is not imported, this fails.
Merge pull request #2757 from iMichka/v1.3.x

Added missing warnings import
BUG : makes Axes.margins work with just kwargs

applied fix supplied by @syoung-m4 in #2510
Merge pull request #2762 from tacaswell/margin_arg_counting

BUG : makes Axes.margins work with just kwargs
Merge pull request #2705 from mdboom/fix-numpy-1.9

Build fails on OS X with NumPy 1.9
Conflicts:
	src/file_compat.h
	src/ft2font.cpp
Merge remote-tracking branch 'upstream/v1.3.x'

Conflicts:
	lib/matplotlib/axes/_axes.py