[LNT] Python 3 support: enable testing for Python 3

Summary:
Now that all tests are passing in Python 3 enable testing for Python 3
in addition of testing for Python 2.

Reviewers: cmatthews, hubert.reinterpretcast, kristof.beyls, leandron, PrzemekWirkus

Reviewed By: cmatthews

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D69052

Author: Thomas Preud'homme

tox.ini

@@ -11,6 +11,7 @@ whitelist_files = ./setup.py
 
 [tox]
 envlist = py27
+          py3
           mypy
           flake8
 
@@ -31,6 +32,7 @@ deps =
 commands =
     # No where close to passing yet, but nice to have.
     - mypy --junit-xml=junit-{envname}.xml --py2 --ignore-missing-imports lnt
+    - mypy --junit-xml=junit-{envname}.xml --ignore-missing-imports lnt
 
 [testenv:flake8]
 skip_install = true