Support for root file input directly #468

Job	Run time
conda install (ubuntu-20.04)	5m 31s
Build documentation	2m 51s
conda install latest tensorflow (ubuntu-20.04)	15m 35s
Unit tests (ubuntu-20.04, 3.7)	17m 50s
Unit tests (macos-11, 3.7)	26m 41s
Unit tests (ubuntu-20.04, 3.6)	23m 33s
Unit tests (ubuntu-20.04, 3.8)	14m 41s
Test uproot3 (ubuntu-20.04, 3.7)	14m 6s
Style checks	2m 56s
Coverage (ubuntu-20.04, 3.7)	15s
	2h 3m 59s

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