Add Python3 testing support

Conditionally runs the tests depending on the availability of Python
versions (because Travis is the worst).
diff --git a/tools/run_tests/jobset.py b/tools/run_tests/jobset.py
index 8694b8f..1278e77 100755
--- a/tools/run_tests/jobset.py
+++ b/tools/run_tests/jobset.py
@@ -81,6 +81,7 @@
 
 _TAG_COLOR = {
     'FAILED': 'red',
+    'WARNING': 'yellow',
     'TIMEOUT': 'red',
     'PASSED': 'green',
     'START': 'gray',