[pre-commit.ci] pre-commit autoupdate

updates:
- [github.com/asottile/pyupgrade: v2.20.0 → v2.21.0](asottile/pyupgrade@v2.20.0...v2.21.0)

Author: pre-commit-ci[bot]

.pre-commit-config.yaml

@@ -27,7 +27,7 @@ repos:
     -   id: reorder-python-imports
         args: [--py3-plus]
 -   repo: https://github.com/asottile/pyupgrade
-    rev: v2.20.0
+    rev: v2.21.0
     hooks:
     -   id: pyupgrade
         args: [--py36-plus]