Add pyupgrade hook to pre-commit config

Inspired by Nova's patch [1].

[1]: https://review.opendev.org/c/openstack/nova/+/896987

Change-Id: I7f8d2e820d539949e1724de7b82d685415ee3cd3

.pre-commit-config.yaml

@@ -30,3 +30,8 @@ repos:
       entry: flake8
       files: '^.*\.py$'
       exclude: '^(doc|releasenotes|tools)/.*$'
+- repo: https://github.com/asottile/pyupgrade
+  rev: v3.13.0
+  hooks:
+    - id: pyupgrade
+      args: [--py38-plus]