Stringification of empty bound parameters fails with exception

[moshez]

Example:

>>> from clize import parser
>>> a=parser.CliSignature([])      
>>> b=a.read_arguments([], "thing")
>>> str(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/moshez/src/caparg/build/caparg/lib/python3.6/site-packages/attr/_make.py", line 814, in repr_
    for name in attr_names
  File "/home/moshez/src/caparg/build/caparg/lib/python3.6/site-packages/attr/_make.py", line 814, in <genexpr>
    for name in attr_names
AttributeError: 'CliBoundArguments' object has no attribute 'namedparam'

Expected instead useful string return value.

[epsy]

Quick fix in ad57e79, with a cleaner fix pending release in attrs (python-attrs/attrs#308)

[epsy]

Fixed in be37065