Stop vendoring cloudpickle

[gillins]

Just a thought - since there are now regular fixes for new Python versions upstream (see https://github.com/cloudpipe/cloudpickle/commits/master) maybe we should stop trying to vendor our own version and always be out of date. The rate of Python internal changes does seem to be increasing...
Both conda-forge and Ubuntu provide packages for cloudpickle. Of course you can also pip install it so not really hard for a user to obtain.
How about we remove it from rios and expect the user to have it installed if they are using the parallel functionality (users who aren't will see no difference)?

I can modify the conda-forge recipe to install it when rios is installed.

[neilflood]

I agree with this. When we first started doing things with cloudpickle, it was a bit more niche, but it has since become much more widely used. I think we can rely on it to be available from standard sources, if the user wants, and so we can probably stop including our own copy.

Very good, thank you.

[gillins]

I'll submit a PR for this after #61 is merged.