rust-dependencies: update `regex` to 1.3.9 Version `1.3.8` introduces support for empty alternations, which makes previously disallowed patterns usable in `regex`. From a user's perspective, this means that glob patterns like `*.py{,c}` will no longer generate an "invalid" regex and will use the Rust path. `1.3.9` is a bugfix release, might as well update to the latest one. Differential Revision: https://phab.mercurial-scm.org/D8593