Proposal: Backwards-compatible Checkpoint impl

[jaonewguy]

For #186

If the comments don't make sense, you can also read the Effective Python chapter "Item 44: Make pickle Reliable with copyreg"

Tested on scratch and dry run, resume still works as before. Haven't tested with different "versioning" will likely need to do some unit/integration testing for that.

[jaonewguy]

Regarding versioning and package version:

We don't have a setup.py structure like in firstpartylibs right now. If we were to use one of the solutions other than reading in an external file (number 4 in https://packaging.python.org/guides/single-sourcing-package-version/), should we create that structure in a separate task first?

I think the simplest solution for our purposes would be to put that version number in a config or txt file right now.

Some more helpful links for visibility: https://stackoverflow.com/questions/458550/standard-way-to-embed-version-into-python-package https://milkr.io/kfei/5-common-patterns-to-version-your-Python-package/4 https://stackoverflow.com/questions/17583443/what-is-the-correct-way-to-share-package-version-with-setup-py-and-the-package

Found this from enigma-catalyst project: https://github.com/warner/python-versioneer

[ronaldlam]

Yeah we can separate the versioning and this task. There are too many options... So with the simple solution, we need to remember to tag and update versions when deploying. I quite like the setuptools_scm way too. Although if we do go with that I think we need to be careful with the tagging. Regardless, we will need to update how the deployment works.