Updating external dependencies (``cpython-source-deps``)

..note::

Only core developers can push to the ``cpython-source-deps`` repository.

Pull requests are not accepted because the sources must be verified for

integrity. Contributors should create an issue requesting the updated

version and then wait for a core developer to prepare the new version

before proceeding with the next steps below.

Dependencies for Windows CPython builds are `stored in a separate repository

<https://github.com/python/cpython-source-deps>`_ and then fetched during

builds of CPython for Windows in the script:cpy-file:`PCbuild/get_externals.bat`.