Remove readline from test_highline #40

Job	Run time
test (ubuntu-latest, head)	16s
test (ubuntu-latest, 3.3)	13s
test (ubuntu-latest, 3.2)	14s
test (ubuntu-latest, 3.1)	16s
test (ubuntu-latest, 3.0)	15s
test (ubuntu-latest, jruby)	40s
test (ubuntu-latest, jruby-head)	43s
test (ubuntu-latest, truffleruby)	39s
test (ubuntu-latest, truffleruby-head)	45s
test (ubuntu-latest, truffleruby+graalvm)	3m 6s
test (windows-latest, head)	1m 26s
test (windows-latest, 3.3)	2m 1s
test (windows-latest, mingw)	1m 7s
test (windows-latest, mswin)	1m 10s
test (windows-latest, ucrt)	1m 32s
test (macos-latest, head)	30s
test (macos-latest, 3.3)	21s
	15m 14s

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