Merge pull request #1127 from herwinw/thread_variables #1660

Job	Run time
specs (ubuntu, 3.0.6)	1m 30s
RuboCop	23s
specs (ubuntu, 3.1.4)	1m 30s
specs (ubuntu, 3.2.2)	1m 25s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 37s
specs (macos, 3.1.4)	1m 24s
specs (macos, 3.2.2)	1m 18s
specs (macos, 3.3.0)	1m 9s
specs (windows, 3.0.6)	3m 50s
specs (windows, 3.1.4)	4m 44s
specs (windows, 3.2.2)	4m 34s
specs (windows, 3.3.0)	4m 25s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	30m 36s

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