Fixed IndexError when using @staticmethod with *args combination

[anti1869]

In this case with at least Python 3.4.3:

class Foo(object):
    @staticmethod
    def bar(*args):
        """Some docs"""

There is

Exception occurred:
  File "/Users/anti1869/Sites/video/venv-v/lib/python3.4/site-packages/sphinx_autodoc_typehints.py", line 49, in process_signature
    del argspec.args[0]
IndexError: list assignment index out of range

As you can see from the trace, args is empty in FullArgSpec.

FullArgSpec(args=[], varargs='args', varkw=None, defaults=None, kwonlyargs=[], kwonlydefaults=None, annotations={})

So I decided to add simple check.

[agronholm]

Didn't realize staticmethods would still be considered methods. Thanks!