Add GCC pragmas to silence compiler warning about ffi_prep_closure

[pablogsal]

This silences this warning also for GCC and clang on Linux:

~/github/python/master master* 1m 20s
❯ make -j -s
 CC='gcc -pthread' LDSHARED='gcc -pthread -shared    ' OPT='-g -Og -Wall'       _TCLTK_INCLUDES='' _TCLTK_LIBS=''       ./python -E ./setup.py -q build
/home/pablogsal/github/python/master/Modules/_ctypes/callbacks.c: In function ‘_ctypes_alloc_callback’:
/home/pablogsal/github/python/master/Modules/_ctypes/callbacks.c:437:9: warning: ‘ffi_prep_closure’ is deprecated: use ffi_prep_closure_loc instead [-Wdeprecated-declarations]
  437 |         result = ffi_prep_closure(p->pcl_write, &p->cif, closure_fcn, p);
      |         ^~~~~~
In file included from /home/pablogsal/github/python/master/Modules/_ctypes/callbacks.c:6:
/usr/include/ffi.h:334:1: note: declared here
  334 | ffi_prep_closure (ffi_closure*,
      | ^~~~~~~~~~~~~~~~

The following modules found by detect_modules() in setup.py, have been
built by the Makefile instead, as configured by the Setup files:
_abc                  atexit                pwd
time

[serhiy-storchaka]

Maybe backport it? Warnings in 3.9 annoy me.

[miss-islington]

Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[miss-islington]

Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-23590 is a backport of this pull request to the 3.9 branch.

[miss-islington]

Sorry, @pablogsal, I could not cleanly backport this to 3.8 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker cce3f0b0c88eba98bc11abe703a444bee7880ff8 3.8