What GOG is attempting with Galaxy 2.0 [0] is interesting in this department. Galaxy 2.0 has plugins to import what it can of games/achievements/play time from as many other vendors as it can and tries to show something of a view of every game across all your major services/accounts. Many of those plugins are open source, written in Python, though of course Galaxy 2.0 itself is not itself open source and it is still the primary installer/launcher for GOG's own service, if you are concerned about vendor control. There's a third party export script in Python as well [1], though it is reading JSON and SQLite files directly and not using an official API.

[0] https://www.gog.com/galaxy

[1] https://github.com/AB1908/GOG-Galaxy-Export-Script