Drop dependency on Python "future" package

It's not widely packaged in Linux distributions, and none of the VIPS
Python code requires Python 3 semantics from any of the builtins future
was replacing.

python/README.md

@@ -33,10 +33,3 @@ or
     sudo cp Vips.py /usr/lib/python3/dist-packages/gi/overrides
 
 You can optionally pre-compile this file for a small speedup.
-
-For python2.7, you need to install the "future" package. In Ubuntu, for
-example, you can do something like:
-
-    sudo apt-get install python-pip
-    sudo pip install future
-

python/Vips.py

@@ -3,21 +3,6 @@
 
 from __future__ import division
 
-# if this is py2 and future is not found, it's the first import from builtins 
-# that fails silently, for some reason 
-
-# raise a RuntimeError, this is not caught by our importer
-
-try:
-    from builtins import map
-except Exception as e:
-    raise RuntimeError("Unable to import 'map' from 'builtins': " + str(e) + 
-                       " -- maybe try 'pip install future'")
-
-from builtins import str
-from builtins import range
-from builtins import object
-
 # overrides for pygobject gobject-introspection binding for libvips, tested 
 # with python2.7 and python3.4
 

test/test_arithmetic.py

@@ -1,8 +1,6 @@
 #!/usr/bin/python3
 
 from __future__ import division
-from builtins import zip
-from builtins import range
 import unittest
 import math
 

test/test_colour.py

@@ -1,7 +1,5 @@
 #!/usr/bin/python
 
-from builtins import zip
-
 import unittest
 import math
 

test/test_conversion.py

@@ -1,7 +1,6 @@
 #!/usr/bin/python
 
 from __future__ import division
-from builtins import zip
 import unittest
 import math
 

test/test_convolution.py

@@ -1,8 +1,6 @@
 #!/usr/bin/python3
 
 from __future__ import division
-from builtins import zip
-from builtins import range
 from numbers import Number
 from functools import reduce