MAINT: Add python_requires to package metadata

[effigies]

Should probably have done this during the release.

[effigies]

Preserving the test result, as I rebase out the Circle branch criterion change:

$ #!/bin/bash -eo pipefail
pyenv install 3.3.7
pyenv local 3.3.7
FAIL=false
pip install dist/nipype-*-py2.py3-none-any.whl || FAIL=true
$FAIL

Downloading Python-3.3.7.tar.xz...
-> https://www.python.org/ftp/python/3.3.7/Python-3.3.7.tar.xz
Installing Python-3.3.7...
Installing pip from https://bootstrap.pypa.io/3.3/get-pip.py...
Installed Python-3.3.7 to /opt/circleci/.pyenv/versions/3.3.7

DEPRECATION: Python 3.3 supported has been deprecated and support for it will be dropped in the future. Please upgrade your Python.
Processing ./dist/nipype-1.2.2.dev0+g2033d7a25-py2.py3-none-any.whl
nipype requires Python '>= 2.7, != 3.0.*, != 3.1.*, != 3.2.*, != 3.3.*' but the running Python is 3.3.7

[effigies]

Canceling Circle because there's no real need for it here. IMO we can merge when Travis passes.