gnu: python-sphinx-panels: Use python-sphinx@4.

...also add deprecation notice, and change its users accordingly.

* gnu/packages/sphinx.scm (python-sphinx-panels)[propagated-inputs]: Change
from PYTHON-SPHINX to PYTHON-SPHINX-4.
[description]: Mention that it is unmaintained.
* gnu/packages/python-science.scm (python-scipy)[native-inputs]: Likewise.
* gnu/packages/python-xyz.scm (python-numpy-documentation)[native-inputs]: Likewise.

gnu/packages/python-science.scm

@@ -146,7 +146,7 @@ (define-public python-scipy
            python-pydata-sphinx-theme
            python-pytest
            python-pytest-xdist
-           python-sphinx
+           python-sphinx-4
            python-sphinx-panels
            python-threadpoolctl
            which))

gnu/packages/python-xyz.scm

@@ -5731,7 +5731,7 @@ (define-public python-numpy-documentation
            python-pandas
            python-pydata-sphinx-theme
            python-scipy                 ;used by matplotlib
-           python-sphinx
+           python-sphinx-4
            python-sphinx-panels
            texinfo
            texlive-bin

gnu/packages/sphinx.scm

@@ -377,14 +377,16 @@ (define-public python-sphinx-panels
            (lambda* (#:key tests? #:allow-other-keys)
              (when tests?
                (invoke "pytest")))))))
-    (propagated-inputs (list python-docutils python-sphinx))
+    (propagated-inputs (list python-docutils python-sphinx-4))
     (native-inputs
      (list python-pytest
            python-pytest-regressions))
     (home-page "https://github.com/executablebooks/sphinx-panels")
     (synopsis "Sphinx extension for creating panels in a grid layout")
     (description
-     "This package provides a sphinx extension for creating panels in a grid layout.")
+     "This package provides a sphinx extension for creating panels in a
+grid layout.  It is no longer maintained and users are encouraged to use
+@code{sphinx-design} instead.")
     (license license:expat)))
 
 (define-public python-sphinxcontrib-programoutput