numpydoc.docscrape: Restore support for Python 3.4.

Fixing backwards compatibility bug introduced in commit
254494c, which broke support for Python 3.4 when passing
the builtin method 'mro' of 'type' objects to
'inspect.signature', raising a 'ValueError'.

Author: the-real-droop

numpydoc/docscrape.py

@@ -473,9 +473,9 @@ def __init__(self, func, role='func', doc=None, config={}):
         if not self['Signature'] and func is not None:
             func, func_name = self.get_func()
             try:
-                if hasattr(inspect, 'signature'):
+                try:
                     signature = str(inspect.signature(func))
-                else:
+                except (AttributeError, ValueError):
                     # try to read signature, backward compat for older Python
                     if sys.version_info[0] >= 3:
                         argspec = inspect.getfullargspec(func)