in py35+ you'll also want to include kwonlyargs

Since this creates a disparate branch you'll probably want to include # pragma: no cover comments on each branch

Maybe something like this?

argspec = getargspec(lambda_)
if PY2:  # pragma: no cover
    argspec = inspect.getargspec(lambda_)
    unsuported = (...)
    unsupported += (getattr(argspec, 'kwonlyargs', None),)
else:  # pragma: no cover
    argspec = inspect.getfullargspec(lambda_)
    unsupported_argspec_attrs = (argspec.varags, argspec.keywords, argspect.defaults)

if unsupported_argspec_attrs: ...

Note that I picked a subtly different form that uses if PY2: else: -- this makes the python3 branch continue to work in python4 (if it were ever to come out)

kwonlyargs is not supported on getargspec, it's something that was introduced with getfullargspec. I'm not entirely clear what this function is used for, I just went for feature-parity. What would kwonlyargs add over varkw?

oh oops, I was in a hurry, that should only be in the python3 case (A newly added feature)

def kwonly_func(x, *, y=None):
    """y may only be passed as  a kwarg"""