Bump regex-syntax from 0.6.19 to 0.6.20

Bumps [regex-syntax](https://github.com/rust-lang/regex) from 0.6.19 to 0.6.20.
- [Release notes](https://github.com/rust-lang/regex/releases)
- [Changelog](https://github.com/rust-lang/regex/blob/master/CHANGELOG.md)
- [Commits](rust-lang/regex@regex-syntax-0.6.19...regex-syntax-0.6.20)

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