Update source-date-epoch Python code snippet (?)

The documentation for versions of Python from 3.8 to-date include a cautionary note in the datetime module to indicate that the datetime.utcfromtimestamp is no-longer-recommended, and that datetime.fromtimestamp is preferred.

One of the Python code snippets that we include on the SOURCE_DATE_EPOCH documentation page uses datetime.utcfromtimestamp -- so we may want to upgrade/migrate that. I'll try to look at this soon.