python: Fix pickling support for Python 3.11

The addition of object.__getstate__ in Python 3.11(*) broke logic in
the pickling support code that assumed that __getstate__ would only
exist on a wrapped class if it were explicitly provided. Under
3.11+, this code now ignores __getstate__ if it came from object
and not the class itself.

This maintains behavior prior to Python 3.11 when pickling instances
with attributes added directly to their __dict__. A new test case was
added to pickle1 to exercise this behavior. This also fixes other
failures in pickle1 and pickle4 where __reduce__ would otherwise
include an unexpected "None" entry in the tuple it returns.

(*) https://docs.python.org/3/whatsnew/3.11.html#other-language-changes

Fixes #3384

(Internal change: 2351569)

pxr/external/boost/python/src/object/pickle_support.cpp

@@ -48,6 +48,25 @@ namespace {
       }
       result.append(initargs);
       object getstate = getattr(instance_obj, "__getstate__", none);
+
+      // Python 3.11 added a default implementation of __getstate__ to
+      // object, which we need to ignore to maintain previous behavior.
+      // 
+      // For example, if a class had pickle support enabled but did not
+      // provide __getstate__, instances with items directly added to their
+      // __dict__ would skip the __getstate_manages_dict__ safeguard below
+      // prior to 3.11. After 3.11, the safeguard would be triggered.
+#if PY_VERSION_HEX >= 0x030b00f0
+      if (!getstate.is_none()) {
+          object class_getstate = getattr(instance_class, "__getstate__", none);
+          handle<> obj_getstate(PyObject_GetAttrString(
+              (PyObject*)&PyBaseObject_Type, "__getstate__"));
+          if (class_getstate.ptr() == obj_getstate.get()) {
+              getstate = none;
+          }
+      }
+#endif
+
       object instance_dict = getattr(instance_obj, "__dict__", none);
       long len_instance_dict = 0;
       if (!instance_dict.is_none()) {

pxr/external/boost/python/test/pickle1.py

@@ -24,6 +24,15 @@
     >>> print(wl.greet())
     Hello from California!
 
+    >>> wd = pickle1_ext.world('Emeryville')
+    >>> wd.zipcode = 94608
+    >>> pstr = pickle.dumps(wd)
+    >>> wl = pickle.loads(pstr)
+    >>> print(wd.greet(), "({})".format(wd.zipcode))
+    Hello from Emeryville! (94608)
+    >>> print(wl.greet(), "({})".format(wl.zipcode))
+    Hello from Emeryville! (94608)
+
     >>> noop = pickle1_ext.noop()
     >>> try: pickle.dumps(noop)
     ... except RuntimeError as e: print(str(e)[:55])