Remove Python 2.7 compatibility

[agurgel-te]

Summary

Fixes #95.

• Remove example/python2
• Make travis and tox pyenvs match setup.py
• Revert to built-in open in setup.py
• Remove caps on transitive dependencies in setup.py

I thought of including a change to fully fix tox, but it seems unused and I didn't want to do too many unrelated things in the PR. Anyway, after the following diff, tox shows a passing result:

diff --git a/tox.ini b/tox.ini
index afdfad4..29d9a49 100644
--- a/tox.ini
+++ b/tox.ini
@@ -14 +13,0 @@ deps =
-    -r{toxinidir}/requirements.txt

Requirements (place an x in each [ ])

• [X] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [X] I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[seratch]

Hi @antonio-te, thanks for taking the time to make this contribution!

• Would you mind signing our CLA? Without having it, we are unable to have your changes in this repo
• You're right about tox. It is no longer used and we are happy to remove related files. But I agree that we can have minimum changes in this PR. The current fix looks great to me.

Once your fix is merged, we will release a new version - 3.0.0.

[agurgel-te]

Would you mind signing our CLA? Without having it, we are unable to have your changes in this repo

I wouldn't mind (I read it top-to-bottom, in fact), but I haven't had the occasion to sign one during the course of my work so I'm checking with my team to make doubly sure it's A-OK.

[agurgel-te]

Apologies for the delay; I've gained approval to sign the CLA, and did so just now.

[seratch]

@antonio-te Thanks for working on this! After cleaning up a bit more, we will release v3.0 shortly.