Update unit3.ipynb

Update (python 3.9)

notebooks/unit3/unit3.ipynb

@@ -289,7 +289,9 @@
       },
       "outputs": [],
       "source": [
-        "!pip install -r requirements.txt"
+        "!pip install -r requirements.txt",
+        "# Since colab uses Python 3.9 we need to add this installation\n",
+        "!pip install gym[atari,accept-rom-license]==0.21.0"
       ]
     },
     {