The discussion on [1] was for removing pure Python implementation,
not about changing C implementation.

[1]: https://mail.python.org/pipermail/python-dev/2017-September/149147.html

While I withdrawed my suggestion about removing pure Python implementation,
I still think this new implementation is valuable.

Dict ordering is not language spec and many libraries including stdlib
uses OrderedDict to keep insertion order.
Faster creation, iteration and 1/2 memory usage is good enhancement
for most use cases.