Test with tox

Simpler and more predictable than self-managing a virtualenv. I've set
it up to allow arguments to be passed through to `pytest` by running
e.g. `tox -- -f`.

Author: adamchainz

.coveragerc

@@ -0,0 +1,2 @@
+[run]
+omit = .tox/*

pytest.ini

@@ -1,4 +1,4 @@
 [pytest]
 looponfailroots = .
-addopts = --cov .
+addopts = --cov-config .coveragerc --cov .
 #addopts = --cov . --cov-report=html -f

tox.ini

@@ -0,0 +1,6 @@
+[tox]
+envlist = py27
+
+[testenv]
+deps = -rtest-requirements.txt
+commands = pytest {posargs}