Add python and pip to Dockerfile to support running 20.04 notebooks o…

…n deepnote.com

The current images will not run on deepnote.  Adding these lines fulfills the requirements, as specified here:
https://docs.deepnote.com/environment/custom-environments#custom-image-requirements

To reproduce, I have uploaded the revised image to DockerHub as robotlocomotion/drake:pr15586-focal.  Visit https://deepnote.com/project/Drake-PR-15586-mPXpOqUZTqe8JBtfj5z4xQ/%2Fnotebook.ipynb .  You will need to create an account to proceed, I'm afraid.  Then
- On the left menu, click on the Environment toolbar, and then look at the "Environment" dropdown.
- I've connected 3 docker images: robotlocomotion/drake:bionic and robotlocomotion/drake:focal and robotlocomotion/drake:pr15586-focal
- Confirm that selecting the two existing images will spend some time "Loading image..." but then will fail with

`We failed to run the image you specified. Your image doesn’t exist, or we don’t have permissions to access it, or it doesn't fulfill the requirements to run.`

- Confirm that selecting the revised image will load successfully.
- For the revised image, run the first cell of the notebook, and see that pydrake loads successfully and prints the expected file path.

tools/install/dockerhub/focal/Dockerfile

@@ -7,6 +7,9 @@ RUN export DEBIAN_FRONTEND=noninteractive \
   && apt-get update -qq \
   && apt-get install --no-install-recommends -o Dpkg::Use-Pty=0 -qy \
     $(cat /opt/drake/share/drake/setup/packages-focal.txt) \
+# Make python and pip available to meet the requirements for deepnote.com:
+# https://docs.deepnote.com/environment/custom-environments#custom-image-requirements
+    python3-pip python-is-python3 \
   && rm -rf /opt/drake/share/drake/setup /var/lib/apt/lists/* \
   && mv /opt/drake/bin/drake-visualizer /opt/drake/bin/drake-visualizer.image
 RUN printf '#!/bin/bash\n \