Update python-slugify requirement from <7,>=6 to >=6,<9

Updates the requirements on [python-slugify](https://github.com/un33k/python-slugify) to permit the latest version.
- [Changelog](https://github.com/un33k/python-slugify/blob/master/CHANGELOG.md)
- [Commits](un33k/python-slugify@v6.0.0...v8.0.1)

---
updated-dependencies:
- dependency-name: python-slugify
  dependency-type: direct:production
...

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

setup.py

@@ -80,7 +80,7 @@
         "click>=7,<9",
         "furl>=2,<3",
         "subprocess-tee>=0.3,<1",
-        "python-slugify>=6,<7",
+        "python-slugify>=6,<9",
         "tld>=0.10,<1",
     ],
     extras_require={