bpo-31338 (#3374)

* Add Py_UNREACHABLE() as an alias to abort().
* Use Py_UNREACHABLE() instead of assert(0)
* Convert more unreachable code to use Py_UNREACHABLE()
* Document Py_UNREACHABLE() and a few other macros.

Modules/_tracemalloc.c

@@ -726,7 +726,7 @@ tracemalloc_realloc(void *ctx, void *ptr, size_t new_size)
 
                The GIL and the table lock ensures that only one thread is
                allocating memory. */
-            assert(0 && "should never happen");
+            Py_UNREACHABLE();
         }
         TABLES_UNLOCK();
     }