Python: Fix GC support for callbacks

Make the references to the callbacks visible to the GC
to be able to break cycles.

api/python/tests/test_gc.py

@@ -0,0 +1,41 @@
+# Copyright © SixtyFPS GmbH <info@slint.dev>
+# SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.1 OR LicenseRef-Slint-commercial
+
+from slint import slint as native
+import weakref
+import gc
+
+
+def test_callback_gc():
+    compiler = native.ComponentCompiler()
+
+    compdef = compiler.build_from_source("""
+        export component Test {
+            out property <string> test-value: "Ok";
+            callback test-callback(string) -> string;
+        }
+    """, "")
+    assert compdef != None
+
+    instance = compdef.create()
+    assert instance != None
+
+    class Handler:
+        def __init__(self, instance):
+            self.instance = instance
+
+        def python_callback(self, input):
+            return input + instance.get_property("test-value")
+
+    handler = Handler(instance)
+    instance.set_callback(
+        "test-callback", handler.python_callback)
+    handler = None
+
+    assert instance.invoke("test-callback", "World") == "WorldOk"
+
+    wr = weakref.ref(instance)
+    assert wr() is not None
+    instance = None
+    gc.collect()
+    assert wr() is None