Get multiprocessing working in kitty

Monkeypatch the stdlib multiprocessing module to
use kitty as its python interpreter for spawning worker
processes.

kitty_tests/tui.py

@@ -42,3 +42,7 @@ class TestTUI(BaseTest):
         self.ae(le.cursor_pos, 0)
         le.backspace()
         self.assertTrue(le.pending_bell)
+
+    def test_multiprocessing_spawn(self):
+        from kitty.multiprocessing import test_spawn
+        test_spawn()