Update python sample code (#12681)

sample-code/python/README.md

@@ -12,6 +12,14 @@ pip install -r requirements.txt
 py.test test
 ```
 
+To run them on Sauce Labs, make sure that you have the following environment
+variables set:
+```
+SAUCE_LABS
+SAUCE_USERNAME
+SAUCE_ACCESS_KEY
+```
+
 ### Run an arbitrary file
 
 ```