Since things are piling up, here's a quick record of what I think the remaining tasks are: (in approximate order of priority)
1. Finish writing docs (is updating library/typing.html sufficient? https://github.com/python/cpython/pull/32103)
2. Implement support for pickling of unpacked native tuples
3. Implement support and add tests for copy() of TypeVarTuple and unpacked tuple
4. Resolve the issue of how we implement type substitution (https://bugs.python.org/issue47006)