[Mne_analysis] Dropping support for Python 2

[Brunner, Clemens (clemens.brunner@uni-graz.at)]

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Hello!

Recently, I brought up the question when MNE should transition from supporting
both Python 2 and 3 to only supporting Python 3 (https://github.com/mne-tools/m
ne-python/issues/5167). Many packages have already pledged to drop support for
Python 2 no later than January 1st 2020. Most of these packages, including
NumPy and SciPy, will drop Python 2 support much earlier. For example, NumPy
will support only Python 3 starting with January 1st 2019 (so the last Python 2
compatible NumPy version will be released before December 31st 2018) (https://d
ocs.scipy.org/doc/numpy-1.14.0/neps/dropping-python2.7-proposal.html).

I think this is a good idea and MNE should follow this roadmap:
- Support Python 2 and 3 until December 31st 2018.
- Starting January 1st 2019, all new major releases will only support Python 3.
- Bugfix releases to the last Python 2 release (likely 0.17) will be made until
December 31st 2019.

I'm not sure how much work the last point is, if it is too much this period
could be shortened to e.g. mid-2019.

This plan means that MNE 0.17 will be the last release that runs on Python 2
(there might be some bugfix releases 0.17.1, 0.17.2, ... after that). Starting
with MNE 0.18, only Python 3 will be supported.

If you want to leave a comment, please feel free to reply to this message or
post at https://github.com/mne-tools/mne-python/issues/5167.

Clemens