Remove 3.1 on CI #1953

Job	Run time
specs (ubuntu, 3.2.7)	1m 31s
RuboCop	50s
specs (ubuntu, 3.3.7)	1m 28s
specs (ubuntu, 3.4.2)	1m 34s
specs (macos, 3.2.7)	1m 8s
specs (macos, 3.3.7)	1m 23s
specs (macos, 3.4.2)	1m 36s
specs (windows, 3.2.7)	4m 22s
specs (windows, 3.3.7)	4m 19s
specs (windows, 3.4.2)	4m 19s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 29s
	23m 59s

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