adding a _typing module

this is a place that *must* only be imported inside a if
typing.TYPE_CHECKING block. It is a mixture of smoothing over thinigs
that moved from typing_extensions to typing in later python versions and
useful but typing only code.

This makes typing_extensions required for python versions older than
3.8 *when running mypy*. typing_extensions should *only* be imported
inside an `if typing.TYPE_CHECKING` block (include the new _typing.py
module) to ensure that it doesn't become a runtime dependency

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