tox: combine coverage reports for each version of Python

tox.ini

@@ -12,12 +12,17 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test_requirements.txt
     coverage
+whitelist_externals =
+    mv
 commands =
     # run tests both with and without the C extensions (including Cython)
     {envpython} setup.py --quiet build_ext --inplace
     {envbindir}/coverage run -m nose2 --no-user-config tests
+    mv .coverage .coverage.with_extensions
     {envpython} -c 'import os; [os.remove(os.path.join(d, fn)) for (d, dns, fns) in os.walk("{envdir}") for fn in fns if fn.endswith(".so")]'
     {envbindir}/coverage run -m nose2 --no-user-config tests
+    mv .coverage .coverage.no_extensions
+    {envbindir}/coverage combine
 
 [testenv:py26]
 deps =