>> One, we should not blindly pull in the PyPy code
>> without some core PyPy developer being in on this

You can count me among the PyPy developers.

> I concur.  Much of PyPy code is written for a restricted subset of
> Python instead of clean, idiomatic modern Python.

Not this part. The module datetime.py is meant to be imported by the interpreter, and has no limitation (we call it "application-level" code, opposed to interpreter-level code which is translated to C and which indeed has serious constraints)