Add pyupgrade pre-commit hook.

I'm targeting pretty recent Python versions (3.6+) so why not enjoy the
latest syntax and features.

.pre-commit-config.yaml

@@ -90,6 +90,11 @@ repos:
         additional_dependencies:
           - toml
 
+  - repo: https://github.com/asottile/pyupgrade
+    rev: v2.23.2
+    hooks:
+      - id: pyupgrade
+
   - repo: local
     hooks:
       - id: poetry check