Bump pur from 5.4.3 to 6.0.0

Bumps [pur](https://github.com/alanhamlett/pip-update-requirements) from 5.4.3 to 6.0.0.
- [Release notes](https://github.com/alanhamlett/pip-update-requirements/releases)
- [Changelog](https://github.com/alanhamlett/pip-update-requirements/blob/master/HISTORY.rst)
- [Commits](alanhamlett/pip-update-requirements@5.4.3...6.0.0)

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

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

requirements-dev.txt

@@ -8,7 +8,7 @@ docformatter==1.4
 coverage==6.2
 autoflake==1.4
 tox==3.24.5
-pur==5.4.3
+pur==6.0.0
 wheel==0.37.1
 setuptools==60.5.0
 twine==3.7.1