Bump pytest-runner from 5.1 to 6.0.0

Bumps [pytest-runner](https://github.com/pytest-dev/pytest-runner) from 5.1 to 6.0.0.
- [Release notes](https://github.com/pytest-dev/pytest-runner/releases)
- [Changelog](https://github.com/pytest-dev/pytest-runner/blob/main/CHANGES.rst)
- [Commits](pytest-dev/pytest-runner@5.1...v6.0.0)

---
updated-dependencies:
- dependency-name: pytest-runner
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

requirements_dev.txt

@@ -13,4 +13,4 @@ Sphinx==2.2.0
 sphinx_rtd_theme==0.4.3
 bumpversion==0.5.3
 watchdog==0.9.0
-pytest-runner==5.1
+pytest-runner==6.0.0