Use tox for running tests and building docs

* Set up tox for testing supported Django/Python combinations
* Also include tox env for checking and building docs
* Use tox-travis for Travis CI integration
* Add tests against Django master
* Document building docs and running tests with tox

docs/_readme/docutils.cfg

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+# docutils (rst2html) config for generating static HTML that approximates
+# PyPI package description rendering (as of 3/2018).
+#
+# Usage (in package root dir):
+#   python setup.py --long-description | rst2html.py --config=docs/_readme/docutils.cfg > ${OUTDIR}/readme.html
+#
+# Requires docutils and pygments (both are installed with Sphinx)
+
+[general]
+# Duplicate docutils config used by PyPA readme_renderer.
+# https://github.com/pypa/readme_renderer/blob/master/readme_renderer/rst.py
+# (This isn't exactly what's used on legacy PyPI, but it's close enough.)
+cloak_email_addresses = True
+doctitle_xform = True
+sectsubtitle_xform = True
+initial_header_level = 2
+file_insertion_enabled = False
+math_output = MathJax
+raw_enabled = False
+smart_quotes = True
+strip_comments = True
+syntax_highlight = short
+
+# Halt rendering and throw an exception if there was any errors or warnings from docutils.
+halt_level = 2
+# DON'T Disable all system messages from being reported.
+#       (We're not running inside readme_renderer, so *do* want to see warnings and errors.)
+# report_level = 5
+
+# Approximate PyPI's styles:
+stylesheet = docs/_readme/readme.css

docs/_readme/readme.css

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+/*
+    readme.css
+    Approximates PyPI package description rendering as of 3/2018,
+    using docutils rst2html output.
+ */
+
+/* Borrow base docutils and pygments styles directly from PyPI: */
+@import url("https://pypi.python.org/static/css/docutils.css");
+@import url("https://pypi.python.org/static/css/pygments.css"); /* requires rst2html 'short' classnames */
+
+/* Subset of PyPI site styles applicable to package description: */
+HTML, BODY {
+    font-family: Arial, Verdana, Geneva, "Bitstream Vera Sans", Helvetica, sans-serif;
+    font-size: 103%;
+    color: #000;
+    background-color: #FFF;
+}
+H1, H2, H3, H4, H5 {
+    font-family: Georgia, "Bitstream Vera Serif", "New York", Palatino, serif;
+    font-weight: normal;
+    line-height: 1em;
+}
+H1 {
+    font-size: 160%;
+    color: #234764;
+    margin: 0.7em 0;
+    text-decoration: none;
+}
+H2 {
+    font-size: 140%;
+    color: #366D9C;
+    margin: 0.7em 0 0.7em 0;
+}
+IMG {
+    border: 0;
+}
+A:link {
+    color: #00A;
+}
+A:visited {
+    color: #551A8B;
+}
+P A:link, P A:visited,
+UL A:link, UL A:visited,
+OL A:link, OL A:visited {
+    text-decoration: none;
+    border-bottom: 1px dashed #ccc;
+}
+
+/* Additional styles, to account for not having all of PyPI's wrapper and navigation divs: */
+body {
+    line-height: 1.5;
+    font-size: 14.6px; /* ~computed font-size in PyPI's div#content */
+}
+h1.title {
+    text-align: left;
+}
+ul, li {
+    margin-left: 1em;
+    padding-left: 0;
+}
+pre {
+    padding: 10px;
+    font-size: 11.9px; /* ~computed font-size in PyPI's div#content pre */
+}
+
+/* Give the page a little breathing room: */
+.document {
+    max-width: 960px;
+    margin: 0 auto;
+}
+body {
+    padding: 1em;
+}