Improves second python notebook example

[discdiver]

Updated the second python notebook to:

• use more common pandas methods.
• use pandas to plot (with matplotlib under the hood) instead of plotly. This results in fewer dependencies. Plotly is great, but it takes more work to install and get set up. Running the existing notebook from top to bottom in JupyterLab, the plot didn't render for me with the latest version of plotly.
• remove source argument to wbdata.get_dataframe() so the API call will succeed instead of erroring.