Extensions: clarify the identifiers must "Python" id's in the error

Without this, it's not clear what a "valid identifier" means,
ref: #136517.

scripts/addons_core/bl_pkg/cli/blender_ext.py

@@ -1531,7 +1531,7 @@ def url_retrieve_exception_as_message(
 
 def pkg_idname_is_valid_or_error(pkg_idname: str) -> str | None:
     if not pkg_idname.isidentifier():
-        return "Not a valid identifier"
+        return "Not a valid Python identifier"
     if "__" in pkg_idname:
         return "Only single separators are supported"
     if pkg_idname.startswith("_"):