provide provide a decorator to alias keyword kwargs into `_` suffix alternatives

[graingert]

provide a decorator to alias keyword kwargs into _ suffix alternatives

@six.pep492
def task1(ham, async_): return async_

@six.pep492
def task2(ham, await_): return await_

so that

ob = object()
assert (
    ob
    is task1(async=ob)
    is task1(ob)
    is task1(async_=ob)
    is task2(await=ob)
    is task2(ob)
    is task2(await_=ob)
)

See also amyreese/fissix#15