Issue 36106: resolve sinpi() name clash with libm

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)

Hi!, 

It seem a good improve to resolve a potential problem. But I am not convinced if that problem could appear because sinpi() is definded locally.

This has a potential header conflict. For the same reason functions like
lgamma, etc. are prefixed with `m_` in Modules/mathmodule.c

e.g. if I rename `m_lgamma` to `lgamma` I get
```
mathmodule.c:389:1: error: static declaration of ‘lgamma’ follows non-static declaration
 lgamma(double x)
```

New changeset f57cd8288dbe6aba99c057f37ad6d58f8db75350 by Serhiy Storchaka (Dima Pasechnik) in branch 'master':
bpo-36106: Resolve sinpi name clash with libm (IEEE-754 violation). (GH-12027)
https://github.com/python/cpython/commit/f57cd8288dbe6aba99c057f37ad6d58f8db75350

New changeset 4e6646fef5d2cc53422e4eca0b18201ed5a5c4b6 by Miss Islington (bot) in branch '3.7':
bpo-36106: Resolve sinpi name clash with libm (IEEE-754 violation). (GH-12027)
https://github.com/python/cpython/commit/4e6646fef5d2cc53422e4eca0b18201ed5a5c4b6

Downstream issue: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232792

New changeset b545ba0a508a5980ab147ed2641a42be3b31a2db by Serhiy Storchaka (Dima Pasechnik) in branch '2.7':
[2.7] bpo-36106: resolve sinpi name clash with libm (IEEE-754 violation). (GH-12027) (GH-12050)
https://github.com/python/cpython/commit/b545ba0a508a5980ab147ed2641a42be3b31a2db

This one was my fault. Thanks for the fix!

Can this also be backported to 3.5. and 3.6, as requested by FreeBSD maintainers?

I'll nosy Ned as the release manager about the request to 3.6.

messages: + msg336633
versions: + Python 2.7, Python 3.8

stage: patch review