Use os._exit in the child instead of sys.exit.

src/portpicker.py

diff --git a/src/portpicker.py b/src/portpicker.py
index 10b373d..a887781 100644
--- a/src/portpicker.py
+++ b/src/portpicker.py
@@ -280,7 +280,7 @@
                     for held_socket in bound_sockets:
                         held_socket.close()
                 finally:
-                    sys.exit(0)
+                    os._exit(0)
 
 
 def _pick_unused_port_without_server(bind_timeout=0):