bpo-31979: Simplify transforming decimals to ASCII (#4336)

in int(), float() and complex() parsers.

This also speeds up parsing non-ASCII numbers by around 20%.
diff --git a/Objects/complexobject.c b/Objects/complexobject.c
index 4bcf2ce..2c886c7 100644
--- a/Objects/complexobject.c
+++ b/Objects/complexobject.c
@@ -914,10 +914,10 @@
         if (s_buffer == NULL) {
             return NULL;
         }
+        assert(PyUnicode_IS_ASCII(s_buffer));
+        /* Simply get a pointer to existing ASCII characters. */
         s = PyUnicode_AsUTF8AndSize(s_buffer, &len);
-        if (s == NULL) {
-            goto exit;
-        }
+        assert(s != NULL);
     }
     else {
         PyErr_Format(PyExc_TypeError,
@@ -928,7 +928,6 @@
 
     result = _Py_string_to_number_with_underscores(s, len, "complex", v, type,
                                                    complex_from_string_inner);
-  exit:
     Py_DECREF(s_buffer);
     return result;
 }