[scikit-image] Python 3 transition

[Stefan van der Walt]

Hi, everyone

I'd like to make a 4th attempt to determine our transition pathway to
Python 3.  The last thread is here:

https://mail.python.org/pipermail/scikit-image/2017-July/005304.html

I would like to propose that the next version, 0.14 (scheduled for March
of next year), becomes the last targeted for Python 2.  It will also be
an LTS (Long Time Support), which means we will backport serious fixes
for a period of one year.  0.15 will then target Python >= 3.5 only
(sorry, no f-strings yet!).

Let me know your thoughts.

Stéfan