Merge pull request #140 from jiangyi15/histogram_fill #625

Job	Run time
Build documentation	2m 7s
conda install (ubuntu-20.04)	2m 49s
conda install latest tensorflow (ubuntu-20.04)	4m 27s
Unit tests (ubuntu-20.04, 3.7)	16m 16s
Unit tests (macos-11, 3.7)	34m 48s
Test uproot3 (ubuntu-20.04, 3.7)	15m 41s
Unit tests (ubuntu-20.04, 3.6)	14m 6s
Style checks	2m 14s
Unit tests (ubuntu-20.04, 3.8)	15m 2s
Coverage (ubuntu-20.04, 3.7)	17s
	1h 47m 47s

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