tests: Treat ResourceWarning as an error

This will hopefully prevent new cases creeping in.

Note: ResourceWarning doesn't exist in Python 2.x, it was added in Python 3.2.

Author: moreati

tox.ini

@@ -110,6 +110,7 @@ setenv =
     NOCOVERAGE_ERASE = 1
     NOCOVERAGE_REPORT = 1
     PIP_CONSTRAINT={toxinidir}/tests/constraints.txt
+    PYTHONWARNINGS=error::ResourceWarning,default
     # Only applicable to MODE=mitogen
     distro_centos5: DISTRO=centos5
     distro_centos6: DISTRO=centos6