fix custom signal handling, which was broken after #166

There was a typo in #181: when the pari sigint handling was converted
to a general mechanism, the line

```
    PARI_SIGINT_pending = 0;
```

got translated into

```
    custom_signal_unblock();
```
instead of the correct
```
    custom_set_pending_signal(0);
```

This error didn't take effect until #166 removed the pari sigint
handling.

This causes some doctest failures in sagemath:
```
src/sage/coding/linear_code.py
src/sage/geometry/integral_points.pxi
src/sage/rings/integer.pyx
src/sage/rings/polynomial/polynomial_element.pyx
```
related to mishandling of AlarmInterrupt.

See: https://github.com/sagemath/cysignals/pull/181/files#r1904885037

src/cysignals/implementation.c

@@ -591,7 +591,7 @@ static void _sig_on_interrupt_received(void)
     do_raise_exception(cysigs.interrupt_received);
     cysigs.sig_on_count = 0;
     cysigs.interrupt_received = 0;
-    custom_signal_unblock();
+    custom_set_pending_signal(0);
 
 #if HAVE_SIGPROCMASK
     sigprocmask(SIG_SETMASK, &oldset, NULL);