"properties" unit test fails with Python 3.11

[sunyab]

The "properties" unit test fails when boost::python is built against Python 3.11 or later. I've pasted in the test output below (note: the output mentions 3.12, but I've tested this against 3.11 as well).

I believe this failure is due to Python now including more detail in tracebacks -- see https://docs.python.org/3/whatsnew/3.11.html#whatsnew311-pep657

====== BEGIN OUTPUT ======
running...
**********************************************************************
File "/root/boost_1_86_0/libs/python/test/properties.py", line 56, in __main__
Failed example:
    x1.value_r = 2
Expected:
    Traceback (most recent call last):
      File "properties.py", line 49, in ?
        x1.value_r = 2
    AttributeError: can't set attribute
Got:
    Traceback (most recent call last):
      File "/usr/lib/python3.12/doctest.py", line 1361, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest __main__[16]>", line 1, in <module>
        x1.value_r = 2
        ^^^^^^^^^^
    AttributeError: property of 'X' object has no setter
**********************************************************************
1 items had failures:
   1 of  28 in __main__
***Test Failed*** 1 failures.

EXIT STATUS: 1
====== END OUTPUT ======

This was originally reported here: PixarAnimationStudios/OpenUSD#3384