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/utils.py

@@ -38,7 +38,7 @@ def get_migration_abspath(app_label, migration_name):
     from django.db.migrations.loader import MigrationLoader
 
     module_name, _ = MigrationLoader.migrations_module(app_label)
-    migration_path = "{}.{}".format(module_name, migration_name)
+    migration_path = f"{module_name}.{migration_name}"
     migration_module = import_module(migration_path)
 
     migration_file = migration_module.__file__