backwards compatibility fix for pdm <2.26.2 (#32)

Author: sigma67

src/pdm_build_locked/command.py

@@ -44,8 +44,9 @@ def handle(self, project: Project, options: argparse.Namespace) -> None:
             super().handle(project, options)
             return
 
-        # we need to let pdm known that we're intending to write this file
-        project.pyproject.open_for_write()
+        # we need to let pdm known that we're intending to write this file (only for pdm versions >=2.26.2)
+        if hasattr(project.pyproject, "open_for_write"):
+            project.pyproject.open_for_write()
 
         # we are not interested in the pdm dev-dependencies group
         pdm_dev_dependencies = set()