ValueError for some signature

[tristanlatr]

Describe the bug
docspec_python is crashing on some signature.

To Reproduce
Steps to reproduce the behavior:

1. Try to parse a function with the following signature : (x, y, /), it crashes.

Expected behavior
A clear and concise description of what you expected to happen.

Traceback

signature = '(x, y, /)', mod_from_text = <function _docspec_python.mod_from_text at 0x10e802ca0>

>   ???

pydocspec/pydocspec/test/test_astbuilder.py:608: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pydocspec/pydocspec/test/test_astbuilder.py:585: in test_function_signature
    ???
pydocspec/test/__init__.py:71: in mod_from_text
    docspec_modules = list(_docspec_python.load_python_modules(
pydocspec/test/__init__.py:66: in load_python_modules
    yield docspec_python.parse_python_module(f, filename='<fromtext>',
.tox/test/lib/python3.8/site-packages/docspec_python/__init__.py:133: in parse_python_module
    return parser.parse(ast, filename, module_name)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:110: in parse
    member = self.parse_declaration(module, node)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:128: in parse_declaration
    return self.parse_funcdef(parent, node, False, decorations)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:257: in parse_funcdef
    args = self.parse_parameters(parameters)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:350: in parse_parameters
    node = index.next()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <nr.util.iter._sequence_walker.SequenceWalker object at 0x11bd15160>

    def next(self) -> T:
      """ Returns the next element in the sequence. Note that this cannot be used to advance the walker beyond the
      end of the sequence; use #advance() instead. """
      if self.has_next():
        self.index += 1
        return self.sequence[self.index]
      else:
>       raise ValueError('no next element for the walker to move to')
E       ValueError: no next element for the walker to move to

.tox/test/lib/python3.8/site-packages/nr/util/iter/_sequence_walker.py:37: ValueError
____________________________________ test_function_signature_posonly[mod_from_text0-(x, y=0, /)] _____________________________________

signature = '(x, y=0, /)', mod_from_text = <function _docspec_python.mod_from_text at 0x10e802ca0>

>   ???

pydocspec/pydocspec/test/test_astbuilder.py:608: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pydocspec/pydocspec/test/test_astbuilder.py:585: in test_function_signature
    ???
pydocspec/test/__init__.py:71: in mod_from_text
    docspec_modules = list(_docspec_python.load_python_modules(
pydocspec/test/__init__.py:66: in load_python_modules
    yield docspec_python.parse_python_module(f, filename='<fromtext>',
.tox/test/lib/python3.8/site-packages/docspec_python/__init__.py:133: in parse_python_module
    return parser.parse(ast, filename, module_name)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:110: in parse
    member = self.parse_declaration(module, node)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:128: in parse_declaration
    return self.parse_funcdef(parent, node, False, decorations)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:257: in parse_funcdef
    args = self.parse_parameters(parameters)
.tox/test/lib/python3.8/site-packages/docspec_python/parser.py:350: in parse_parameters
    node = index.next()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <nr.util.iter._sequence_walker.SequenceWalker object at 0x11bdfa3a0>

    def next(self) -> T:
      """ Returns the next element in the sequence. Note that this cannot be used to advance the walker beyond the
      end of the sequence; use #advance() instead. """
      if self.has_next():
        self.index += 1
        return self.sequence[self.index]
      else:
>       raise ValueError('no next element for the walker to move to')
E       ValueError: no next element for the walker to move to

.tox/test/lib/python3.8/site-packages/nr/util/iter/_sequence_walker.py:37: ValueError

[NiklasRosenstein]

Hey @tristanlatr, seems like this was fixed some time along the way? I've added a test for your particular case in #76.