bpo-36106: Resolve sinpi name clash with libm (IEEE-754 violation). (GH-12027)

The standard math library (libm) may follow IEEE-754 recommendation to
include an implementation of sinPi(), i.e. sinPi(x):=sin(pi*x).
And this triggers a name clash, found by FreeBSD developer
Steve Kargl, who worken on putting sinpi into libm used on FreeBSD
(it has to be named "sinpi", not "sinPi", cf. e.g.
https://en.cppreference.com/w/c/experimental/fpext4

).
(cherry picked from commit f57cd8288dbe6aba99c057f37ad6d58f8db75350)

Co-authored-by: Dima Pasechnik <dimpase@gmail.com>