Python version compatibility metadata

[twm]

This PR adds packaging metadata that marks the package as only supporting Python 3.5+. This prevents Pip from installing it on Python 2.7.

You can find documentation here: https://setuptools.readthedocs.io/en/latest/setuptools.html#new-and-changed-setup-keywords (search in page for "python_requires").

I'm not sure how you do releases, but I'll note that you may need non-ancient (last four years or so) setuptools to generate a wheel that contains this metadata.

Fixes #131.

[coveralls]

Coverage remained the same at ?% when pulling 7b09005 on twm:py2-setup-metadata into c7f0a80 on hamcrest:master.

[coveralls]

Coverage remained the same at ?% when pulling fe00696 on twm:py2-setup-metadata into c7f0a80 on hamcrest:master.

[coveralls]

Coverage remained the same at ?% when pulling fe00696 on twm:py2-setup-metadata into c7f0a80 on hamcrest:master.

[coveralls]

Coverage remained the same at ?% when pulling fe00696 on twm:py2-setup-metadata into c7f0a80 on hamcrest:master.

[coveralls]

Coverage remained the same at ?% when pulling fe00696 on twm:py2-setup-metadata into c7f0a80 on hamcrest:master.

[offbyone]

Not quite the point; the 1.x series should support Python 2, not the other way around.

[markrwilliams]

Not quite the point; the 1.x series should support Python 2, not the other way around.

python_requires corresponds to PEP 345's Requires-Python field:

This field specifies the Python version(s) that the distribution is guaranteed to be compatible with.
https://www.python.org/dev/peps/pep-0345/#requires-python

@twm's usage is correct.

[twm]

Oh, I see: #130 (comment)

So should I retarget this at some other branch?

[offbyone]

@markrwilliams I know their usage is correct, but it's not the right change.

@twm arguably yes, but honestly that needs to end up in the ball of commits that's gonna get yanked out and re-inserted into the 2.0 line. I'm inclined to let @brunns sort that stuff out and then see if this is still necessary then. If so, definitely re-target it there.

[twm]

Okay, I'll leave it as-is for now. Let me know if there is anything I need to do. If it's obsolete feel free to close — I won't be offended. Thanks for PyHamcrest!

[brunns]

I'm pretty sure we want this on master, at least, so I'll merge it. We wouldn't want it on any Python 2 compatible releases, but any such release would have to be cut from a much older commit.