Fix _CountingAttr hack with attrs master

[benbovy]

A workaround before (hopefully) a cleaner solution (see python-attrs/attrs#340).

Re-use argument list from attr.attrib for _CountingAttr.__init__(). It should at least reduce the risk of breaking the code when new arguments are introduced.

[benbovy]

Monkey patching _CountingAttr is probably better, after all (but still a hack). It adds the compute decorator for all attr.ib and not just for on_demand variables, but we could just ignore it when we don't need it.