docs: kernel_include.py: Cope with docutils 0.21

Running "make htmldocs" on a newly installed Sphinx 7.3.7 ends up in
a build error:

Sphinx parallel build error:
AttributeError: module 'docutils.nodes' has no attribute 'reprunicode'

docutils 0.21 has removed nodes.reprunicode, quote from release note [1]:

* Removed objects:

docutils.nodes.reprunicode, docutils.nodes.ensure_str()
Python 2 compatibility hacks

Sphinx 7.3.0 supports docutils 0.21 [2]:

kernel_include.py, whose origin is misc.py of docutils, uses reprunicode.

Upstream docutils removed the offending line from the corresponding file
(docutils/docutils/parsers/rst/directives/misc.py) in January 2022.
Quoting the changelog [3]:

Deprecate `nodes.reprunicode` and `nodes.ensure_str()`.

Drop uses of the deprecated constructs (not required with Python 3).

Do the same for kernel_include.py.

Tested against:
- Sphinx 2.4.5 (docutils 0.17.1)
- Sphinx 3.4.3 (docutils 0.17.1)
- Sphinx 5.3.0 (docutils 0.18.1)
- Sphinx 6.2.1 (docutils 0.19)
- Sphinx 7.2.6 (docutils 0.20.1)
- Sphinx 7.3.7 (docutils 0.21.2)

Link: http://www.docutils.org/RELEASE-NOTES.html#release-0-21-2024-04-09 [1]
Link: https://www.sphinx-doc.org/en/master/changes.html#release-7-3-0-released-apr-16-2024 [2]
Link: https://github.com/docutils/docutils/commit/c8471ce47a24 [3]
Signed-off-by: Akira Yokosawa <[e-mail]>
Cc: [e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/sphinx/kernel_include.py

···
         # HINT: this is the only line I had to change / commented out:
         #path = utils.relative_path(None, path)
 
-        path = nodes.reprunicode(path)
         encoding = self.options.get(
             'encoding', self.state.document.settings.input_encoding)
         e_handler=self.state.document.settings.input_encoding_error_handler