gtk-doc: add and default to python 3.13 variant

remove python 3.8

gnome/gtk-doc/Portfile

@@ -83,32 +83,32 @@ proc py_setup {p_py_ver} {
                     port:py${py_ver_nodot}-mock
 }
 
-variant python38 conflicts python39 python310 python311 python312 description {Build using Python 3.8} {
-    py_setup 3.8
-}
-
-variant python39 conflicts python38 python310 python311 python312 description {Build using Python 3.9} {
+variant python39 conflicts python310 python311 python312 python313 description {Build using Python 3.9} {
     py_setup 3.9
 }
 
-variant python310 conflicts python38 python39 python311 python312 description {Build using Python 3.10} {
+variant python310 conflicts python39 python311 python312 python313 description {Build using Python 3.10} {
     py_setup 3.10
 }
 
-variant python311 conflicts python38 python39 python310 python312 description {Build using Python 3.11} {
+variant python311 conflicts python39 python310 python312 python313 description {Build using Python 3.11} {
     py_setup 3.11
 }
 
-variant python312 conflicts python38 python39 python310 python311 description {Build using Python 3.12} {
+variant python312 conflicts python39 python310 python311 python313 description {Build using Python 3.12} {
     py_setup 3.12
 }
 
-if {![variant_isset python38] &&
-    ![variant_isset python39] &&
+variant python313 conflicts python39 python310 python311 python312 description {Build using Python 3.13} {
+    py_setup 3.13
+}
+
+if {![variant_isset python39] &&
     ![variant_isset python310] &&
     ![variant_isset python311] &&
-    ![variant_isset python312] } {
-    default_variants +python312
+    ![variant_isset python312] &&
+    ![variant_isset python313] } {
+    default_variants +python313
 }
 
 # some tests are known to fail in gtk-doc 1.29+