install sphinx-proof for google collab compat

Author: mmcky

Diff for: lectures/cass_fiscal.md

@@ -13,6 +13,14 @@ kernelspec:
 
 # Cass-Koopmans Model with Distorting Taxes 
 
+
+```{code-cell} ipython
+---
+tags: [remove-cell]
+---
+!pip install sphinx-proof
+```
+
 ## Overview
 
 This lecture studies effects of foreseen   fiscal and technology shocks on competitive equilibrium prices and quantities in a nonstochastic version of a Cass-Koopmans  growth model with features described in this QuantEcon lecture {doc}`cass_koopmans_2`.