Drop Python 2.7 testing on CI and update docs for Python 3.5+ support.

[grlee77]

The release notes for 1.0 mention that 1.0.x is the last version with Python 3 support. This PR removes a few mentions of Python 2.7 from the docs and removes the Python 2.7 test cases from the Travis and Appveyor configurations.

There is also some mention of Python 2.7/3.2 in a section on building with MinGW. I was not familiar enough with the current status of MinGW builds on Windows to update that section, so I have left it as is for now.

[rgommers]

Good idea, it's time:)

I suggest to add NUMPYSPEC="numpy==1.13.3" in the py35 or py36 build matrix entry - that should be our new minimum supported version. Right now this PR tests only against the latest released numpy.

[grlee77]

Okay, good suggestion. I have added 1.13.3 to the 3.5 build on Travis and replaced 1.9 with 1.13.3 both the docs and setup.py.

[grlee77]

This PR has removed testing on Python 2, but would not have broken anything for Python 2 users. Do you also want to remove Python 2 specific code at this point?

I only found a couple of things:
1.) we define string_types for handling basestr vs str in pywt/_utils.py
2.) there is a check for the futures module in the test_concurrent.py.

Removing the first of these would definitely break things on Python 2.

[rgommers]

Do you also want to remove Python 2 specific code at this point?

I'd say let's not do that. I don't think it adds much value, and we don't want to make life harder for backports of people that still depend on 2.x - we just don't want to spend effort on it anymore.

[rgommers]

Merged, thanks @grlee77