fix(python): user defined __jsii_proxy_class attributes are not preserved (#4625)

In #4611, we added the `_jsii_proxy_class__` attributes to the `@jsii.interface` implementations. This was required in order to comply with `typeguard` protocol checking. We didn't implement it correctly, accidentally overriding user defined proxy classes.

## Note

I have been wrecking my brain trying to understand if this bug has any runtime implications, and I couldn't find any.

#### How so?

At runtime, from what I could gather, the `__jsii_proxy_class__` attribute is only used when we try to instantiate a subclass of an abstract class:

https://github.com/aws/jsii/blob/dc77d6c7016bcb7531f6e374243410f969ea1fbf/packages/%40jsii/python-runtime/src/jsii/_reference_map.py#L65-L70

However, for abstract classes, we assign an explicit value to `__jsii_proxy_class__`:

https://github.com/aws/jsii/blob/dc77d6c7016bcb7531f6e374243410f969ea1fbf/packages/jsii-pacmak/lib/targets/python.ts#L1496-L1501

Luckily, this happens **AFTER** the `@jsii.implements` decorator has finished, thus overriding the mistake in the decorator.
Presumably, this would still be a problem for user defined abstract classes (since they don't have this assignment). However, reference resolving for user defined classes is done via native reference lookup:

https://github.com/aws/jsii/blob/dc77d6c7016bcb7531f6e374243410f969ea1fbf/packages/%40jsii/python-runtime/src/jsii/_reference_map.py#L48-L54

This is also why I couldn't come up with a real life test case, and had to resort to an artificial one. 

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

Author: iliapolo

packages/@jsii/python-runtime/src/jsii/_runtime.py

@@ -168,7 +168,7 @@ def implements(*interfaces: Type[Any]) -> Callable[[T], T]:
     def deco(cls):
         cls.__jsii_type__ = getattr(cls, "__jsii_type__", None)
         cls.__jsii_ifaces__ = getattr(cls, "__jsii_ifaces__", []) + list(interfaces)
-        cls.__jsii_proxy_class__ = lambda: getattr(cls, "__jsii_proxy_class__", None)
+        cls.__jsii_proxy_class__ = getattr(cls, "__jsii_proxy_class__", lambda: None)
 
         # https://github.com/agronholm/typeguard/issues/479
         cls.__protocol_attrs__ = getattr(cls, "__protocol_attrs__", [])

packages/@jsii/python-runtime/tests/test_python.py

@@ -27,6 +27,37 @@ def test_inheritance_maintained(self):
 
         assert base_names == ["DerivedStruct", "MyFirstStruct"]
 
+
+class TestImplementsInterface:
+
+    def test_jsii_proxy_class_defaults_to_none(self) -> None:
+        @jsii.implements(IBaz)
+        class MyBaz:
+            pass
+
+        klass = getattr(MyBaz, "__jsii_proxy_class__")()
+        assert klass == None
+
+    def test_jsii_proxy_class_preserves_user_defined_attribute(self) -> None:
+
+        class _MyBazProxy:
+            def baz_method(self) -> str:
+                return "_MyBazProxy"
+
+        @jsii.implements(IBaz)
+        class MyBaz:
+
+            @staticmethod
+            def __jsii_proxy_class__():
+                return _MyBazProxy
+
+            def baz_method(self) -> str:
+                return "MyBaz"
+
+        klass = getattr(MyBaz, "__jsii_proxy_class__")()
+        instance = klass()
+        assert instance.baz_method() == "_MyBazProxy"
+
     def test_implements_interface(self) -> None:
         """Checks that jsii-generated classes correctly implement the relevant jsii-generated interfaces."""