doc: update RTD/tox to Python 3.13

I have some unexplained issue with Python 3.12, updating fixes it.

Author: bluetech

.readthedocs.yaml

@@ -17,7 +17,7 @@ build:
   os: ubuntu-24.04
   tools:
     python: >-
-      3.12
+      3.13
   apt_packages:
     - inkscape
   jobs:

tox.ini

@@ -111,7 +111,7 @@ setenv =
 description =
     build the documentation site under \
     `{toxinidir}{/}doc{/}en{/}_build{/}html` with `{basepython}`
-basepython = python3.12 # sync with rtd to get errors
+basepython = python3.13 # Sync with .readthedocs.yaml to get errors.
 usedevelop = True
 deps =
     -r{toxinidir}/doc/en/requirements.txt