Use python 3.5 for make html and make doctest

[sonots]

To avoid cyfunction is not a Python function as:

/work/cupy/docs/source/reference/generated/cupy.cuda.Event.rst:53: WARNING: error while formatting arguments for cupy.cuda.Event.record: <cyfunction Event.record at 0x2aea1bcb8d10> is not a Python function

It seems insepect.isfunction(cyfuction) returns True from python 3.4 https://groups.google.com/forum/#!topic/cython-users/FvcMPk9n2X8. Using python >= 3.4 resolves the issue.

ref. cupy/cupy#306 (comment)

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All of three must be merged together:

• this PR
• Use python 3.5 for doctest chainer#3653
• Use python 3.5 for doctest cupy/cupy#644

[okuta]

CuPy side is ready.

[okuta]

LGTM!