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

Author: florentx

a2x.py

@@ -960,4 +960,4 @@ def to_text(self):
         a2x.load_conf()
         a2x.execute()
     except KeyboardInterrupt:
-        exit(1)
+        sys.exit(1)

tests/testasciidoc.py

@@ -429,7 +429,7 @@ def usage(msg=None):
     if cmd == 'run':
         tests.run(number, backend)
         if tests.failed or tests.warned or tests.skipped:
-            exit(1)
+            sys.exit(1)
     elif cmd == 'update':
         tests.update(number, backend, force=force)
     elif cmd == 'list':