Update tox.ini to create an xml coverage report

Author: rohan-at-sentry

tox.ini

@@ -8,7 +8,7 @@ deps =
     -r{toxinidir}/requirements/test.txt
     -r{toxinidir}/requirements/publish.txt
 setenv =
-    PYTESTARGS = -v -s --tb=long --cov=cloudevents --cov-report term-missing --cov-fail-under=95
+    PYTESTARGS = -v -s --tb=long --cov=cloudevents --cov-report term-missing --cov-report=xml
 commands = pytest {env:PYTESTARGS} {posargs}
 
 [testenv:reformat]