Keeping client and remote python packages in sync

[tomwhite]

Package versions need to match on the client and remote machine. Usually this is not a problem as remote images are built at the same time as the client is installed, but there is nothing to enforce this, and mismatches are possible if either the client or image is rebuilt, which could cause errors.

It would be good to document a good way to freeze/pin dependencies (possibly using pip-tools). This will requires some changes to pass pinned versions to the relevant place for Lithops and Modal.