Standardizing how to handle adding/removing modules from the stdlib

[brettcannon]

I kicked off a discussion at https://discuss.python.org/t/how-do-we-want-to-manage-additions-removals-to-the-stdlib/10681 which has stopped. Having a policy around what it takes to add a module to the stdlib would probably good, as well as removing one (more about what it takes to trigger the deprecation and removal, not the actual deprecation policy since that's handled by PEP 387).