bpo-38707: Fix for multiprocessing.Process MainThread.native_id (GH-17088)

bpo-38707: Fix for multiprocessing.Process MainThread.native_id (GH-17088)

This PR implements a fix for `multiprocessing.Process` objects; the error occurs when Processes are created using either `fork` or `forkserver` as the `start_method`.

In these instances, the `MainThread` of the newly created `Process` object retains all attributes from its parent's `MainThread` object, including the `native_id` attribute. The resulting behavior is such that the new process' `MainThread` captures an incorrect/outdated `native_id` (the parent's instead of its own).

This change forces the Process object to update its `native_id` attribute during the bootstrap process.

cc @vstinner

https://bugs.python.org/issue38707

Automerge-Triggered-By: @pitrou
(cherry picked from commit c6b20be85c0de6f2355c67ae6e7e578941275cc0)

Co-authored-by: Jake Tesler <jake.tesler@gmail.com>