Update plumbum requirement from <1.7,>=1.6.1 to >=1.6.1,<1.9

Updates the requirements on [plumbum](https://github.com/tomerfiliba/plumbum) to permit the latest version.
- [Release notes](https://github.com/tomerfiliba/plumbum/releases)
- [Changelog](https://github.com/tomerfiliba/plumbum/blob/master/CHANGELOG.rst)
- [Commits](tomerfiliba/plumbum@v1.6.1...v1.8.2)

---
updated-dependencies:
- dependency-name: plumbum
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

setup.py

@@ -138,7 +138,7 @@
 
     'firmware': [
         'GitPython>=2.0.5,<3',
-        'plumbum>=1.6.1,<1.7',
+        'plumbum>=1.6.1,<1.9',
     ],
 
 }