In fact it looks like PyPy implemented it first [0], and it was an implementation detail in 3.6, but declared part of the standard in 3.7 [1].

[0]: https://docs.python.org/3.6/whatsnew/3.6.html#new-dict-imple...

[1]: https://docs.python.org/3.7/whatsnew/3.7.html