Yet some nitpicks.
Currently the code of _PyLong_FromNbInt() is inlined and the do_decref flag is used to prevent needless change refcounts of int objects (see also issue18797). In proposed patch common code is extracted into the _PyLong_FromNbInt() function and int objects increfed and decrefed. Doesn't it affect a performance? PyLong_As* functions used in arguments parsing in a lot of builtins and their .performance is important.
If the patch slowdowns PyLong_As* functions we perhaps should check PyLong_CheckExact() before calling _PyLong_FromNbInt() and use the do_decref flag.
In general the idea and the patch LGTM.