Error when using interactive bst init

Summary

Interactive bst init fails with a Python error.

Steps to reproduce

1. Run bst init
2. Enter a project name

What is the current bug behavior?

It fails and a Python traceback is printed.

What is the expected correct behavior?

A project is created.

Relevant logs and/or screenshots

Traceback (most recent call last):
  File "/home/tommewett/.local/bin/bst", line 8, in <module>
    sys.exit(cli())
  File "/home/tommewett/.local/lib/python3.5/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/tommewett/Documents/buildstream/src/buildstream/_frontend/cli.py", line 223, in override_main
    standalone_mode=standalone_mode, **extra)
  File "/home/tommewett/.local/lib/python3.5/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/tommewett/.local/lib/python3.5/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/tommewett/.local/lib/python3.5/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/tommewett/.local/lib/python3.5/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/tommewett/.local/lib/python3.5/site-packages/click/decorators.py", line 27, in new_func
    return f(get_current_context().obj, *args, **kwargs)
  File "/home/tommewett/Documents/buildstream/src/buildstream/_frontend/cli.py", line 356, in init
    app.init_project(project_name, format_version, element_path, force, target_directory)
  File "/home/tommewett/Documents/buildstream/src/buildstream/_frontend/app.py", line 369, in init_project
    self._init_project_interactive(project_name, format_version, element_path)
  File "/home/tommewett/Documents/buildstream/src/buildstream/_frontend/app.py", line 859, in _init_project_interactive
    value_proc=project_name_proc, err=True)
  File "/home/tommewett/.local/lib/python3.5/site-packages/click/termui.py", line 126, in prompt
    result = value_proc(value)
  File "/home/tommewett/Documents/buildstream/src/buildstream/_frontend/app.py", line 821, in project_name_proc
    node._assert_symbol_name(None, user_input, 'project name')
  File "src/buildstream/node.pyx", line 1483, in buildstream.node._assert_symbol_name (src/buildstream/node.c:20551)
TypeError: _assert_symbol_name() takes exactly 2 positional arguments (3 given)

---