Add support for Python 3.5+ (#39)

As a software engineer, I'd like to have support for modern versions
of Python declared in setup.py and to have corresponding tests on CI.

Also, I'd like to be able to conveniently run tests
in my local environment, so I want to add a tox config file.

These changes:

 - Add Python 3.5+ to setup.py and .travis.yml

 - Add tox.ini

 - Add a mention of tox into Development section of README

 - Simplify the tests requirements

.travis.yml

@@ -1,6 +1,16 @@
+dist: xenial
+
 language: python
 python:
   - "2.7"
+  - "3.5"
+  - "3.6"
+  - "3.7"
+  - "3.8-dev"
+
+matrix:
+  allow_failures:
+    - python: "3.8-dev"
 
 install:
   - make bootstrap

README.md

@@ -15,6 +15,11 @@ make bootstrap
 make test
 ```
 
+You can use [tox](https://tox.readthedocs.io) to run tests as well.
+```bash
+tox
+```
+
 ### Releases
 
 Before new release, add a summary of changes since last version to CHANGELOG.rst

setup.py

@@ -13,6 +13,9 @@
         'Intended Audience :: Developers',
         'License :: OSI Approved :: MIT License',
         'Programming Language :: Python :: 2.7',
+        'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: Implementation :: PyPy',
         'Topic :: Software Development :: Libraries :: Python Modules',
     ],
@@ -28,10 +31,10 @@
     extras_require={
         'tests': [
             'doubles',
-            'flake8<3',  # see https://github.com/zheller/flake8-quotes/issues/29
+            'flake8',
             'flake8-quotes',
             'mock<1.1.0',
-            'pytest>=2.7,<3',
+            'pytest',
             'pytest-cov',
             'pytest-mock',
             'Sphinx',

tox.ini

@@ -0,0 +1,7 @@
+[tox]
+envlist = py27,py35,py36,py37
+
+[testenv]
+install_command = pip install {opts} {packages} {env:PWD}[tests]
+whitelist_externals = make
+commands =  make test lint