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/sql_analyser/postgresql.py

@@ -16,7 +16,7 @@ def has_create_index(sql_statements, **kwargs):
 
     concerned_table = regex_result.group(2)
     table_is_added_in_transaction = any(
-        sql.startswith("CREATE TABLE {}".format(concerned_table))
+        sql.startswith(f"CREATE TABLE {concerned_table}")
         for sql in sql_statements
     )
     return not table_is_added_in_transaction