[WIP] v3.x - Replace REXML with Nokogiri #417

Job	Run time
rubocop	13s
Unit test (ubuntu-20.04, 3)	1m 29s
Unit test (ubuntu-20.04, 3.1)	1m 49s
Unit test (ubuntu-20.04, 3.2)	1m 40s
Unit test (ubuntu-20.04, 3.3)	1m 37s
Unit test (ubuntu-20.04, jruby-9.4)	2m 12s
Unit test (ubuntu-20.04, truffleruby)	3m 11s
Unit test (macos-latest, 3)	1m 24s
Unit test (macos-latest, 3.1)	1m 42s
Unit test (macos-latest, 3.2)	1m 39s
Unit test (macos-latest, 3.3)	1m 29s
Unit test (macos-latest, jruby-9.4)	1m 51s
Unit test (macos-latest, truffleruby)	2m 31s
Unit test (windows-latest, 3)	3m 19s
Unit test (windows-latest, 3.1)	3m 10s
Unit test (windows-latest, 3.2)	3m 34s
Unit test (windows-latest, 3.3)	4m 48s
finish	0s
	37m 38s

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