Bump pip-tools from 5.5.0 to 6.0.1

Bumps [pip-tools](https://github.com/jazzband/pip-tools) from 5.5.0 to 6.0.1.
- [Release notes](https://github.com/jazzband/pip-tools/releases)
- [Changelog](https://github.com/jazzband/pip-tools/blob/master/CHANGELOG.md)
- [Commits](jazzband/pip-tools@5.5.0...6.0.1)

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

requirements_dev.txt

@@ -5,7 +5,7 @@ coverage[toml]==5.5
 coveralls==3.0.1
 flake8==3.8.4
 furo==2021.2.28b28
-pip-tools==5.5.0
+pip-tools==6.0.1
 pre-commit==2.10.1
 pytest==6.2.2
 pytest-random-order==1.0.4