ENH: restrict to Python 3.5 and higher in setup.py

[nicoa]

Some Foreword: please ask me for further information/MWE if necessary, I hope it will be understandable without for now:

We expercienced some error like the following:

MYENV/local/lib/python2.7/site-packages/pandas/io/common.pyc in writerows(self, rows)
    549             data = self.encoder.encode(data)
    550             # write to the target stream
--> 551             self.stream.write(data)
    552             # empty queue
    553             self.queue.truncate(0)

TypeError: unicode argument expected, got 'str'

This seems to appear in version 0.13.0, pinning back to 0.12.0 seems to work.

Still, referring to the changelog they are both not supported under python 2 (under which this occurred). Is it expected behaviour they get still installed? Is this a problem on pypi side or can it be fixed on the package site?

Thanks for your efforts!

[tswast]

I thought that removing Python 2 from the classifiers and removing the "universal" mark in the manifest would be sufficient (#273) but maybe there's something else we need to do?

[tswast]

What commands are you using to install the package?

[nicoa]

Regarding the commands used: I'm not sure since with no access to it right now, but with rather high probability we install this from a requirements file to some virtualenv using pip. Will update later these days as soon as having access again.

[tswast]

Looks like there's a python_requires we can add to setup.py. https://stackoverflow.com/a/45362425/101923 (docs: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires)

I believe we support Python 3.5 and higher.