Update pre-commit hook adamchainz/blacken-docs to v1.20.0

Author: renovate[bot]

.pre-commit-config.yaml

@@ -23,7 +23,7 @@ repos:
       - id: ruff-format
 
   - repo: https://github.com/adamchainz/blacken-docs
-    rev: 1.19.1 # replace with latest tag on GitHub
+    rev: 1.20.0 # replace with latest tag on GitHub
     hooks:
       - id: blacken-docs
         language: python