Use Python 3.13 on netlify

https://www.netlify.com/blog/announcing-configurable-python-versions-in-netlify-builds/

Author: jarrodmillman

doc/netlify.toml

@@ -1,5 +1,5 @@
 [build.environment]
-  PYTHON_VERSION = "3.8" # netlify currently only support 2.7 and 3.8
+  PYTHON_VERSION = "3.13"
   HUGO_VERSION = "0.139.0"
   DART_SASS_VERSION = "1.81.0"
   DART_SASS_URL = "https://github.com/sass/dart-sass/releases/download/"