Auto-update pre-commit hooks (backport of #347) (#349)

Author: mergify[bot]

.pre-commit-config.yaml

@@ -43,7 +43,7 @@ repos:
 
   # Python hooks
   - repo: https://github.com/asottile/pyupgrade
-    rev: v3.20.0
+    rev: v3.21.0
     hooks:
       - id: pyupgrade
         args: [--py36-plus]