Use standard sys.exit() instead of the fragile exit from site.py module

Rationale:

```bash
$ python -c 'print(type(exit))'
<class 'site.Quitter'>
$ python -Sc 'type(exit)'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
NameError: name 'exit' is not defined