Bump testfixtures from 6.14.0 to 6.18.3

Bumps [testfixtures](https://github.com/Simplistix/testfixtures) from 6.14.0 to 6.18.3.
- [Release notes](https://github.com/Simplistix/testfixtures/releases)
- [Changelog](https://github.com/simplistix/testfixtures/blob/master/CHANGELOG.rst)
- [Commits](simplistix/testfixtures@6.14.0...6.18.3)

---
updated-dependencies:
- dependency-name: testfixtures
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

requirements.txt

@@ -66,7 +66,7 @@ six==1.11.0
     # via
     #   pip-tools
     #   python-dateutil
-testfixtures==6.14.0
+testfixtures==6.18.3
     # via flake8-isort
 toml==0.10.2
     # via