Enable overallocation by default

Author: vstinner

Objects/unicodeobject.c

@@ -13196,6 +13196,7 @@ PyUnicodeWriter_Create(void)
         return NULL;
     }
     _PyUnicodeWriter_Init((_PyUnicodeWriter*)writer);
+    PyUnicodeWriter_SetOverallocate(writer, 1);
     return writer;
 }