fix: relax dask version requirement and use a better check for Task

[lgray]

@martindurant we got pretty... erm direct... feedback that backwards compatibility would be appreciated.

Also the current check was not working with very recent, but older, dask that has Task but it isn't used in blockwise.

[martindurant]

We are accepting more future work by allowing for earlier dask. That must be recognised.

[lgray]

It's less future work w.r.t. people managing clusters and images. There are all sorts of weird regressions from dask version to dask version, their pinning times are much longer than ours.

[lgray]

@martindurant changing things so that we avoid touching the private module _task_spec and grab Task/TaskRef/GraphNode from blockwise. The latter is far less likely to disappear or move, and the naming of the classes is probably pretty stable.

[lgray]

any further commentary on this one?