Use getAvailableCollections() instead of the deprecated .collections (#…

…538)

Co-authored-by: jmcarcell <jmcarcell@users.noreply.github.com>

python/podio/base_writer.py

@@ -51,4 +51,4 @@ def write_frame(self, frame, category, collections=None):
            write. If None, all collections are written
     """
     # pylint: disable-next=protected-access
-    self._writer.writeFrame(frame._frame, category, collections or frame.collections)
+    self._writer.writeFrame(frame._frame, category, collections or frame.getAvailableCollections())