v0.8.21

UnROOT v0.8.21

Diff since v0.8.20

Merged pull requests:

• [RFC] RNTuple reading (#200) (@Moelf)
• CompatHelper: add new compat entry for StructArrays at version 0.6, (keep existing compat) (#201) (@github-actions[bot])
• implement Table.partition and test Arrow round-trips for LazyTree (#202) (@Moelf)
• use macro to cut code repetition in RNTuple (#203) (@Moelf)