Update gnureadline to 6.3.8

[pyup-bot]

There's a new version of gnureadline available.
You are currently using 6.3.3. I have updated it to 6.3.8

These links might come in handy: PyPI | Changelog | Repo

Changelog

6.3.8

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• 42, 44: Address compiler issues (avoid Cygwin, fix multi-arch on gcc)
• 40: Make GPLv3 license explicit
• 39: Look for bash shell in more places
• Uses Python 2.x readline.c from hg 2.7 branch (95814:192f9efe4a38)
• Uses Python 3.x readline.c from hg 3.4 / 3.5 branch (95813:ec6ed10d611e)
• Updated to build against readline 6.3 (patch-level 8)

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖