Cannot install dev dependencies on Python 2.7

[MasterOdin]

This caused the failure in my PR #1193:

IPython 6.0+ does not support Python 2.6, 2.7, 3.0, 3.1, or 3.2.
    When using Python 2.7, please install IPython 5.x LTS Long Term Support version.
    Beginning with IPython 6.0, Python 3.3 and above is required.
    See IPython `README.rst` file for more information:
        https://github.com/ipython/ipython/blob/master/README.rst
    Python sys.version_info(major=2, minor=7, micro=9, releaselevel='final', serial=0) detected.

The dev-requirements therefore would need to be modified (or have a separate dev-requirements for python 2.7), pinning IPython to 5.x so that people could develop for that version?