Update coverage requirement from ^5.1 to ^6.0

Update coverage requirement from ^5.1 to ^6.0

Updates the requirements on [coverage](https://github.com/nedbat/coveragepy) to permit the latest version.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](https://github.com/nedbat/coveragepy/compare/coverage-5.1...6.0)

---
updated-dependencies:
- dependency-name: coverage
  dependency-type: direct:development
...

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