compat.subprocess: Turn on absolute importing.

This prevents the module from importing itself (as opposed to the global
subprocess module) on Python 3 after 2to3 conversion.

lib/matplotlib/compat/subprocess.py

@@ -13,6 +13,7 @@
 This module is safe to import from anywhere within matplotlib.
 """
 
+from __future__ import absolute_import    # Required to import subprocess
 from __future__ import print_function
 
 import subprocess