Fix bound partial for python 3.10

pyro/poutine/messenger.py

@@ -18,6 +18,14 @@ class _bound_partial(partial):
     support class methods as arguments to handlers.
     """
 
+    # Use '__slots__' for func to avoid the issue
+    #   `_bound_partial(_bound_partial(f)).func is f`
+    # in Python 3.10.
+    __slots__ = "func"
+
+    def __init__(self, func):
+        self.func = func
+
     def __get__(self, instance, owner):
         if instance is None:
             return self

tests/poutine/test_poutines.py

@@ -968,3 +968,13 @@ def model():
 
     with poutine.mask(mask=False):
         model()
+
+
+def test_block_class_method():
+    class A:
+        @poutine.block
+        def run(self):
+            return 1
+
+    a = A()
+    a.run()