[Docs] add a new Sphinx VersionChange directive .. scheduled-changed::

[XuehaiPan]

Documentation

Follow the discussion in #127405 (comment):

No reason to actually use the word "deprecation" except to explain what process we're following for changing a user-visible and documented API.

The change is added in PR #131717:

• gh-127405: Emit a deprecation warning about a future change of sys.abiflags availability on Windows #131717

   .. versionchanged:: 3.14
      A deprecation warning will be emitted if the :data:`sys.abiflags` member
      is accessed on Windows.

      The :data:`sys.abiflags` member will be set to a meaningful value on
      Windows in Python 3.16. This means the :data:`sys.abiflags` member will
      always be available on all platforms starting from Python 3.16.

A .. versionchanged:: directive is not very accurate for this change. It would be nice to have a new directive .. scheduled-changed:::

   .. scheduled-changed:: 3.14 3.16
      A deprecation warning will be emitted if the :data:`sys.abiflags` member
      is accessed on Windows.

      The :data:`sys.abiflags` member will be set to a meaningful value on
      Windows in Python 3.16. This means the :data:`sys.abiflags` member will
      always be available on all platforms starting from Python 3.16.

Like the .. deprecated-removed:: directive:

cpython/Doc/tools/extensions/changes.py

Lines 76 to 84 in 8ada7a9

	def setup(app: Sphinx) -> ExtensionMetadata:
	# Override Sphinx's directives with support for 'next'
	app.add_directive("versionadded", PyVersionChange, override=True)
	app.add_directive("versionchanged", PyVersionChange, override=True)
	app.add_directive("versionremoved", PyVersionChange, override=True)
	app.add_directive("deprecated", PyVersionChange, override=True)
	# Register the ``.. deprecated-removed::`` directive
	app.add_directive("deprecated-removed", DeprecatedRemoved)

[AA-Turner]

I'm not sure I see the benefit, I would probably use a caution directive paired with a versionchanged for 3.14.

A

[picnixz]

I also think a caution would be better than a new directive (well, it could help in distinguishing these two when reading the rst but the online docs wouldn't see a difference IMO)