Update black hook repo url

At some point the python GitHub organization renamed to psf (Python Software
Foundation). Although it redirects with no issue, I think it should be updated
to reflect the current repository URL.

.pre-commit-config.yaml

@@ -56,7 +56,7 @@ repos:
       - id: bandit
         args:
           - --config=.bandit.yml
-  - repo: https://github.com/python/black
+  - repo: https://github.com/psf/black
     rev: 20.8b1
     hooks:
       - id: black