py-pyside6: don't create subports based on platform

Changing the version dynamically is bad enough, but changing which
subports exist makes it impossible to list all existing ports without
evaluating every Portfile on every possible platform. As a result,
things like the mirroring job fail if not run on a platform where the
subports are defined.

Instead, always create the subports and set platforms appropriately for
those that do not work on some platforms.

python/py-pyside6/Portfile

@@ -29,7 +29,6 @@ if {[vercmp ${qt6.version} 6.8] >= 0} {
                         sha256  04c712c5b9234add0d9b5aa3c01a0caede64a55b496127751f5d52a230cff748 \
                         size    16669448
 
-    python.versions     310 311 312 313
     set clang_version   19
     # Same clang version as for qt6-qtbase
 } else {
@@ -38,14 +37,19 @@ if {[vercmp ${qt6.version} 6.8] >= 0} {
                         sha256  a4c414be013d5051a2d10a9a1151e686488a3172c08a57461ea04b0a0ab74e09 \
                         size    14591496
 
-    python.versions     39 310 311 312
     set clang_version   18
 }
+python.versions     39 310 311 312 313
 
 distname            pyside-setup-everywhere-src-${version}
 master_sites        https://download.qt.io/official_releases/QtForPython/pyside6/PySide6-${version}-src
 
 if {${name} ne ${subport}} {
+    if {${python.version} >= 313} {
+        platforms   {darwin >= 23}
+    } elseif {${python.version} == 39} {
+        platforms   {darwin <= 22}
+    }
     depends_lib-append \
         port:py${python.version}-numpy