Bump once_cell from 1.5.2 to 1.6.0 (#914)
Bump once_cell from 1.5.2 to 1.6.0 (#914)

Bumps [once_cell](https://github.com/matklad/once_cell) from 1.5.2 to 1.6.0.
- [Release notes](https://github.com/matklad/once_cell/releases)
- [Changelog](https://github.com/matklad/once_cell/blob/master/CHANGELOG.md)
- [Commits](https://github.com/matklad/once_cell/compare/v1.5.2...v1.6.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
alex/ct-tools,alex/ct-tools