chore: Upgrade Python syntax with pyupgrade

Run with the following pre-commit configuration:

```
repos:
  - repo: https://github.com/asottile/pyupgrade
    rev: v3.1.0
    hooks:
      - id: pyupgrade
        args: [ "--py3-plus", "--py36-plus", "--py37-plus" ]
```

The rationale for every change can be found at https://github.com/asottile/pyupgrade

django_migration_linter/management/commands/lintmigrations.py

@@ -19,7 +19,7 @@ from ..utils import (
 CONFIG_NAME = "django_migration_linter"
 PYPROJECT_TOML = "pyproject.toml"
 DEFAULT_CONFIG_FILES = (
-    ".{}.cfg".format(CONFIG_NAME),
+    f".{CONFIG_NAME}.cfg",
     "setup.cfg",
     "tox.ini",
 )