[pre-commit.ci] pre-commit autoupdate

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

.pre-commit-config.yaml

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 69b6d67..4eb0aef 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -27,7 +27,7 @@
     -   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]