Fix Rubocop on 2.x branch #381

Job	Run time
rubocop	13s
Unit test (ubuntu-20.04, 3)	1m 40s
Unit test (ubuntu-20.04, 3.1)	1m 45s
Unit test (ubuntu-20.04, 3.2)	1m 48s
Unit test (ubuntu-20.04, 3.3)	1m 48s
Unit test (ubuntu-20.04, jruby-9.4)	2m 26s
Unit test (ubuntu-20.04, truffleruby)	3m 39s
Unit test (macos-latest, 3)	1m 36s
Unit test (macos-latest, 3.1)	1m 39s
Unit test (macos-latest, 3.2)	1m 37s
Unit test (macos-latest, 3.3)	1m 33s
Unit test (macos-latest, jruby-9.4)	1m 57s
Unit test (macos-latest, truffleruby)	2m 54s
Unit test (windows-latest, 3)	4m 5s
Unit test (windows-latest, 3.1)	2m 54s
Unit test (windows-latest, 3.2)	3m 2s
Unit test (windows-latest, 3.3)	3m 32s
finish	0s
	38m 8s

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