Issue #17098: Make sure every module has __loader__ defined.

Thanks to Thomas Heller for the bug report.
diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c
index 0aac98b..c81de6f 100644
--- a/Modules/signalmodule.c
+++ b/Modules/signalmodule.c
@@ -1367,9 +1367,8 @@
 void
 PyOS_InitInterrupts(void)
 {
-    PyObject *m = PyInit_signal();
+    PyObject *m = PyImport_ImportModule("signal");
     if (m) {
-        _PyImport_FixupBuiltin(m, "signal");
         Py_DECREF(m);
     }
 }