[3.6] bpo-31158: Fix nondeterministic read in test_pty (GH-3808)

[miss-islington]

• bpo-31158: Fix nondeterministic read in test_pty

• Reuse existing readline implementation from io.

Thx to @pitrou

• Updated comment

Ideally, this commit is fixuped into the previous commit. Since there is
already a comment on github, I won't rebase.
(cherry picked from commit e6f62f6)

https://bugs.python.org/issue31158

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LGTM, good bot.

[miss-islington]

Thanks, @Haypo!