build(deps): bump testfixtures from 6.16.0 to 6.17.0

Bumps [testfixtures](https://github.com/Simplistix/testfixtures) from 6.16.0 to 6.17.0.
- [Release notes](https://github.com/Simplistix/testfixtures/releases)
- [Changelog](https://github.com/Simplistix/testfixtures/blob/master/CHANGELOG.rst)
- [Commits](simplistix/testfixtures@6.16.0...6.17.0)

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