Add pyupgrade pre-commit hook (#665)

* Add pyupgrade pre-commit hook

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

.pre-commit-config.yaml

@@ -10,6 +10,11 @@ repos:
   - id: check-toml
   - id: end-of-file-fixer
   - id: trailing-whitespace
+- repo: https://github.com/asottile/pyupgrade
+  rev: v2.23.1
+  hooks:
+  - id: pyupgrade
+    args: [--py36-plus]
 - repo: https://github.com/psf/black
   rev: 21.7b0
   hooks:

src/corsheaders/checks.py

@@ -58,7 +58,7 @@ def check_settings(app_configs, **kwargs):
             allow_all_alias = "CORS_ORIGIN_ALLOW_ALL"
         errors.append(
             checks.Error(
-                "{} should be a bool.".format(allow_all_alias),
+                f"{allow_all_alias} should be a bool.",
                 id="corsheaders.E005",
             )
         )
@@ -71,7 +71,7 @@ def check_settings(app_configs, **kwargs):
     if not is_sequence(conf.CORS_ALLOWED_ORIGINS, str):
         errors.append(
             checks.Error(
-                "{} should be a sequence of strings.".format(allowed_origins_alias),
+                f"{allowed_origins_alias} should be a sequence of strings.",
                 id="corsheaders.E006",
             )
         )