Now is as good a time as any with open PR's at a low.

The recent Python 3 fix adding encoding="utf-8" to the open function
fails on Python 2 because the encoding keyword is not defined. The
solution is to use io.open, which is available for Python 2.6 and 2.7
and is an alias for open when Python >= 3.0. It is reputed to be slow
when running in 2.6, but that should not be a problem.

The default (ascii) encoding breaks:

python3.3 postprocess.py html build/html/*.html
Traceback (most recent call last):
  File "postprocess.py", line 59, in <module>
    main()
  File "postprocess.py", line 27, in main
    lines = process_html(fn, f.readlines())
  File "/usr/lib64/python3.3/encodings/ascii.py", line 26, in

    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in
2309: ordinal not in range(128)

Add `print_function` to all `from __future__ import ...` statements
and use the python3 print function syntax everywhere.

Closes #3078.

The new import `absolute_import` is added the `from __future__ import`
statement and The 2to3 `import` fixer is run to make the imports
compatible. There are several things that need to be dealt with to make
this work.

1) Files meant to be run as scripts run in a different environment than
files imported as part of a package, and so changes to those files need
to be skipped. The affected script files are:

    * all setup.py files
    * numpy/core/code_generators/generate_umath.py
    * numpy/core/code_generators/generate_numpy_api.py
    * numpy/core/code_generators/generate_ufunc_api.py

2) Some imported modules are not available as they are created during
the build process and consequently 2to3 is unable to handle them
correctly. Files that import those modules need a bit of extra work.
The affected files are:

    * core/__init__.py,
    * core/numeric.py,
    * core/_internal.py,
    * core/arrayprint.py,
    * core/fromnumeric.py,
    * numpy/__init__.py,
    * lib/npyio.py,
    * lib/function_base.py,
    * fft/fftpack.py,
    * random/__init__.py

Closes #3172

This should be harmless, as we already are division clean. However,
placement of this import takes some care. In the future a script
can be used to append new features without worry, at least until
such time as it exceeds a single line. Having that ability will
make it easier to deal with absolute imports and printing updates.