Add tox.ini configuration file for tox testing tool

Run multi-python tests with pytest and also static analysis tests with flake8.

tox.ini

@@ -0,0 +1,39 @@
+# Tox (https://tox.readthedocs.io) is a tool for running tests
+# in multiple virtualenvs. This configuration file will run the
+# test suite on all supported python versions. To use it, "pip install tox"
+# and then run "tox" from this directory.
+
+[tox]
+envlist = py27, py34, py35, py36, py37, flake8-27, flake8-37
+minversion = 2.5.0
+skip_missing_interpreters = true
+requires = tox-venv
+
+[testenv]
+deps =
+    -rrequirements.txt
+    pytest
+commands =
+    python -m pytest {posargs}
+extras = test
+
+[testenv:flake8-27]
+basepython = python2.7
+deps = flake8
+commands =
+    flake8 . --count --select=E722,F401,F811,E901, \
+           E999,F821,F822,F823 --show-source --statistics
+    - flake8 . --count --max-complexity=10 \
+             --max-line-length=79 --statistics
+skip_install = True
+
+[testenv:flake8-37]
+basepython = python3.7
+deps = flake8
+commands =
+    flake8 . --count --select=E722,F401,F811,E901, \
+           E999,F821,F822,F823 --show-source --statistics
+    - flake8 . --count --max-complexity=10 \
+             --max-line-length=79 --statistics
+skip_install = True
+