Use venv in simple py test (#1453)

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examples/simple/py/Pulumi.yaml

@@ -1,3 +1,6 @@
 name: simple-random-py
-runtime: python
-description: A simple example of using various random resources in python
+description: A simple example of using various random resources in python
+runtime:
+  name: python
+  options:
+    virtualenv: venv