[pre-commit.ci] pre-commit autoupdate (#363)

Author: pre-commit-ci[bot]

.pre-commit-config.yaml

@@ -16,7 +16,7 @@ repos:
   - id: pyupgrade
     args: [--py36-plus]
 - repo: https://github.com/psf/black
-  rev: 21.8b0
+  rev: 21.9b0
   hooks:
   - id: black
 - repo: https://github.com/asottile/blacken-docs