Merge pull request #1201 from herwinw/stringio_sysread #1772

Job	Run time
RuboCop	31s
specs (ubuntu, 3.0.6)	1m 37s
specs (ubuntu, 3.1.4)	1m 36s
specs (ubuntu, 3.2.2)	1m 30s
specs (ubuntu, 3.3.0)	1m 29s
specs (macos, 3.0.6)	1m 23s
specs (macos, 3.1.4)	1m 23s
specs (macos, 3.2.2)	1m 8s
specs (macos, 3.3.0)	1m 6s
specs (windows, 3.0.6)	4m 22s
specs (windows, 3.1.4)	5m 21s
specs (windows, 3.2.2)	5m 6s
specs (windows, 3.3.0)	4m 35s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 27s
	32m 34s

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