audonnx.Function string representation fails for Python 3.11

[hagenw]

The following works for Python <3.11, but fails for Python 3.11:

>>> import audonnx
>>> object = audonnx.Function(lambda x, sr: float(x.mean()))
>>> print(object)
$audonnx.core.function.Function:
  func: 'lambda x, sr: float(x.mean())'
  func_args: {}
<BLANKLINE>

See #88, where we removed the output from a doctest to make the test work again until this here is fixed.