Python 2.7 is in feature freeze, so 3.7 is the absolute earliest this could be introduced. Given how close we are to 3.7 feature freeze, 3.8 is more likely.

I don't think we would have any objections to supporting hijri calendar, in principle, but as a practical matter I expect that none of the core developers are probably qualified to write, review, support and maintain it. (I could be wrong, of course.) And what interface is required?

I'm not convinced that the datetime module is the right place for this. In the long run, we should expect that Python may support multiple calendars: Western, Arabic, Jewish, Asian calendars, and more. I think that would make datetime too big and clunky.

I think the best approach would be to provide a third-party package on PyPI, and once it has proven itself, it could be proposed for the standard library.