Uproot3

eduardo-rodrigues and chrisburr updated tests for Python 3.9. Thanks!

Added a deprecation warning that points to [scikit-hep/uproot4](https://github.com/scikit-hep/uproot4).

Can finally read unsplit TTrees like small-evnt-tree-nosplit.root and added ROOT 6.20/04 tests.

Fixed 485 with PR 486.

Fixed 465, added support for RNTuple anchor (_only_) 471, finalized jagged array writing 481 and 482.

* PR 466, which fixes issue 465.
* PR 477 for writing jagged arrays to ROOT files (issue 354)
* PR 463, write-update.
* PR 457, docstring formatting.
* PR 450, compression libraries.