…e-sources-md

This commit introduces a Python script that updates the gapminder.json file
in the vega-datasets repository. The script:

- Fetches current data from Gapminder's Google Sheets
- Processes and combines data for life expectancy, population, fertility, and regions
- Filters results to match countries in the existing dataset
- Updates consistent with a minor release

The script maintains data consistency by referencing a specific version
of the existing dataset. This update allows for refreshed Gapminder data
while preserving the column structure and scope of countries/years expected by dependent visualizations.

Related: vega#580