Update sphinx requirements.in and requirements.txt

Update dependencies to fix an urllib3 security advisory and also
to update from Sphinx 8 to Sphinx 9.

Author: daniestevez

docs/requirements.in

@@ -1,2 +1,2 @@
-Sphinx>=8,<9
-sphinx_rtd_theme>=3,<4
+Sphinx>=9,<10
+sphinx_rtd_theme>=3.1,<4

docs/requirements.txt

@@ -1,45 +1,45 @@
 #
-# This file is autogenerated by pip-compile with Python 3.12
+# This file is autogenerated by pip-compile with Python 3.14
 # by the following command:
 #
 #    pip-compile requirements.in
 #
 alabaster==1.0.0
     # via sphinx
-babel==2.17.0
+babel==2.18.0
     # via sphinx
-certifi==2025.11.12
+certifi==2026.2.25
     # via requests
 charset-normalizer==3.4.4
     # via requests
-docutils==0.21.2
+docutils==0.22.4
     # via
     #   sphinx
     #   sphinx-rtd-theme
 idna==3.11
     # via requests
-imagesize==1.4.1
+imagesize==2.0.0
     # via sphinx
 jinja2==3.1.6
     # via sphinx
 markupsafe==3.0.3
     # via jinja2
-packaging==25.0
+packaging==26.0
     # via sphinx
 pygments==2.19.2
     # via sphinx
 requests==2.32.5
     # via sphinx
-roman-numerals-py==3.1.0
+roman-numerals==4.1.0
     # via sphinx
 snowballstemmer==3.0.1
     # via sphinx
-sphinx==8.2.3
+sphinx==9.1.0
     # via
     #   -r requirements.in
     #   sphinx-rtd-theme
     #   sphinxcontrib-jquery
-sphinx-rtd-theme==3.0.2
+sphinx-rtd-theme==3.1.0
     # via -r requirements.in
 sphinxcontrib-applehelp==2.0.0
     # via sphinx
@@ -55,5 +55,5 @@ sphinxcontrib-qthelp==2.0.0
     # via sphinx
 sphinxcontrib-serializinghtml==2.0.0
     # via sphinx
-urllib3==2.6.0
+urllib3==2.6.3
     # via requests