py/objtype: Convert result of user __contains__ method to bool.

Per https://docs.python.org/3/reference/expressions.html#membership-test-operations

    For user-defined classes which define the contains() method, x in y
    returns True if y.contains(x) returns a true value, and False
    otherwise.

Fixes issue #7884.

py/objtype.c

@@ -549,6 +549,7 @@ retry:;
     } else if (dest[0] != MP_OBJ_NULL) {
         dest[2] = rhs_in;
         res = mp_call_method_n_kw(1, 0, dest);
+        res = op == MP_BINARY_OP_CONTAINS ? mp_obj_new_bool(mp_obj_is_true(res)) : res;
     } else {
         // If this was an inplace method, fallback to normal method
         // https://docs.python.org/3/reference/datamodel.html#object.__iadd__ :

tests/basics/class_contains.py

@@ -21,3 +21,27 @@ b = B([1, 2])
 print(1 in b)
 print(2 in b)
 print(3 in b)
+
+
+class C:
+    def __contains__(self, arg):
+        return arg
+
+
+print(C().__contains__(0))
+print(C().__contains__(1))
+print(C().__contains__(''))
+print(C().__contains__('foo'))
+print(C().__contains__(None))
+
+print(0 in C())
+print(1 in C())
+print('' in C())
+print('foo' in C())
+print(None in C())
+
+print(0 not in C())
+print(1 not in C())
+print('' not in C())
+print('foo' not in C())
+print(None not in C())