descriptor.py: fix parsing tr() desc with single leaf TREE

[SomberNight]

tr() descriptors with a single leaf TREE expression cannot be parsed currently:

>>> from hwilib.descriptor import parse_descriptor
>>> b = parse_descriptor("tr(a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,pk(669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0))") 
Traceback (most recent call last):
  File "/home/user/wspace/HWI/hwilib/descriptor.py", line 437, in _get_func_expr
    end = s.rindex(")")
ValueError: substring not found

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/wspace/HWI/hwilib/descriptor.py", line 637, in parse_descriptor
    return _parse_descriptor(desc, _ParseDescriptorContext.TOP)
  File "/home/user/wspace/HWI/hwilib/descriptor.py", line 600, in _parse_descriptor
    subscripts.append(_parse_descriptor(sarg, _ParseDescriptorContext.P2TR))
  File "/home/user/wspace/HWI/hwilib/descriptor.py", line 527, in _parse_descriptor
    func, expr = _get_func_expr(desc)
  File "/home/user/wspace/HWI/hwilib/descriptor.py", line 440, in _get_func_expr
    raise ValueError("A matching pair of parentheses cannot be found")
ValueError: A matching pair of parentheses cannot be found

[achow101]

ACK 4e8559b