bpo-41675: Modernize siginterrupt calls (GH-22028)

siginterrupt is deprecated:

./Modules/signalmodule.c:667:5: warning: ‘siginterrupt’ is deprecated: Use sigaction with SA_RESTART instead [-Wdeprecated-declarations]
  667 |     if (siginterrupt(signalnum, flag)<0) {

Author: pablogsal

Misc/NEWS.d/next/Core and Builtins/2020-08-31-14-53-17.bpo-41675.VSoqWU.rst

@@ -0,0 +1,3 @@
+The implementation of :func:`signal.siginterrupt` now uses :c:func:`sigaction`
+(if it is available in the system) instead of the deprecated :c:func:`siginterrupt`.
+Patch by Pablo Galindo.

Modules/signalmodule.c

@@ -664,7 +664,19 @@ signal_siginterrupt_impl(PyObject *module, int signalnum, int flag)
                         "signal number out of range");
         return NULL;
     }
-    if (siginterrupt(signalnum, flag)<0) {
+#ifdef HAVE_SIGACTION
+    struct sigaction act;
+    (void) sigaction(signalnum, NULL, &act);
+    if (flag) {
+        act.sa_flags &= ~SA_RESTART;
+    }
+    else {
+        act.sa_flags |= SA_RESTART;
+    }
+    if (sigaction(signalnum, &act, NULL) < 0) {
+#else
+    if (siginterrupt(signalnum, flag) < 0) {
+#endif
         PyErr_SetFromErrno(PyExc_OSError);
         return NULL;
     }