I'll note that SciPy actually does have `sinpi` and `cospi`-they just
happen to be private:
https://github.com/scipy/scipy/blob/master/scipy/special/functions.json#L58
https://github.com/scipy/scipy/blob/master/scipy/special/functions.json#L12
They are used extensively inside the module though as helpers in other
special functions and have extensive tests of their own:
https://github.com/scipy/scipy/blob/master/scipy/special/tests/test_mpmath.py#L533
https://github.com/scipy/scipy/blob/master/scipy/special/tests/test_mpmath.py#L547
https://github.com/scipy/scipy/blob/master/scipy/special/tests/test_mpmath.py#L960
https://github.com/scipy/scipy/blob/master/scipy/special/tests/test_mpmath.py#L1741
I have no objections to making them public; the PR is as simple as
removing the underscores and adding a docstring.