Aggregate code coverage data across tox all runs

Fixes #529

docs/changelog.rst

@@ -441,12 +441,16 @@ Core Library
 * `#524 <https://github.com/dw/mitogen/issues/524>` : Python 3.6+ emitted a
   :class:`DeprecationWarning` for :func:`mitogen.utils.run_with_router`.
 
+* `#529 <https://github.com/dw/mitogen/issues/529>` : Code coverage of the
+  test suite was not measured across all Python versions.
+
 
 Thanks!
 ~~~~~~~
 
 Mitogen would not be possible without the support of users. A huge thanks for
 bug reports, testing, features and fixes in this release contributed by
+`Alex Willmer <https://github.com/moreati>`_,
 `Andreas Krüger <https://github.com/woopstar>`_,
 `Anton Stroganov <https://github.com/Aeon>`_,
 `Berend De Schouwer <https://github.com/berenddeschouwer>`_,

tox.ini

@@ -1,10 +1,12 @@
 [tox]
 envlist =
+    init,
     py26,
     py27,
     py35,
     py36,
     py37,
+    report,
 
 [testenv]
 usedevelop = True
@@ -16,6 +18,24 @@ commands =
     {posargs:bash run_tests}
 whitelist_externals =
     bash
+setenv =
+    NOCOVERAGE_ERASE = 1
+    NOCOVERAGE_REPORT = 1
+
+[testenv:init]
+commands =
+    coverage erase
+deps =
+    coverage
+
+[testenv:report]
+commands =
+    coverage html
+    echo "coverage report is at file://{toxinidir}/htmlcov/index.html"
+deps =
+    coverage
+whitelist_externals =
+    echo
 
 [testenv:docs]
 basepython = python