Add python_requires to help pip, and version classifers

[hugovk]

Based on the versions tested by .travis.yml, which match the supported CPython versions.

https://en.wikipedia.org/wiki/CPython#Version_history

And here's the pip installs for terminado from PyPI for May 2018:

python_version	percent	download_count
2.7	41.87%	136,449
3.6	26.07%	84,967
3.5	18.60%	60,618
3.4	13.15%	42,869
3.7	0.27%	876
3.3	0.03%	103
2.6	0.00%	15
3.8	0.00%	1
Total		325,898

Source: pypinfo --start-date 2018-05-01 --end-date 2018-05-31 --percent --markdown terminado pyversion

[hugovk]

The Travis failure is unrelated, latest master is failing in the same way:

https://travis-ci.org/jupyter/terminado/builds/361105398

[takluyver]

As in testpath, I don't like using these specific classifiers. They only get outdated and then need to be updated. If we need to specify Python version compatibility, requires-python is the way to do it.

[takluyver]

My take is that this is more trouble than it's worth until we can express it concisely (>=3.4). But I'm open to other opinions.

The field in flit is called requires-python, though: http://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section

[hugovk]

Thanks, I've removed the x.y classifiers, and corrected this field.

[takluyver]

Thanks. I'll give other people a chance to opine on the use of the requires-python field here.

[takluyver]

The test failure on Py 2.7 seems to be reproducible. :-/

[takluyver]

Close/reopen to rerun tests