avoids ambiguities and implementation problems of iso8601 duration strings

go back to aniso8601 dependency, since we no longer get anything distinctive
from isodate

tahoe-lafs 1.15 depends on attrs<20
something else pulls in attrs 21.2.0
pip fails to resolve the dependencies correctly in this case

So optimistic...

Cannot figure out how to have a VCS or other non-package-name dependency w/ install_requires.

We can put a plugin here later.