protocols/rpc.py:26: DeprecationWarning: 'xdrlib' is deprecated

[Robert]

While tunning pytest on testsuite/test_serial.py (see #386), a deprecation warning was indicated:

============================================================================================================= warnings summary ==============================================================================================================
protocols/rpc.py:26
  /home/robby/builds/pyvisa-py/pyvisa_py/protocols/rpc.py:26: DeprecationWarning: 'xdrlib' is deprecated and slated for removal in Python 3.13
    import xdrlib

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================================================================================================= 1 passed, 1 warning in 0.14s ========================================================================================================

[Matthieu Dartiailh]

xdrlib will be removed from the stdlib since it is considered dead batteries. I was planning to wait and see if a third party copy was emerging.

Otherwise we could simply vendor it (along with tests).