* DatabaseClient: add missing class method `settings_to_cmd_args_env` and add missing arg to `runshell`
The `DatabaseClient.runshell()` was refactored in Django 3.2 to be more standardized
across the different database backends.
* DatabaseClient: make `settings_dict` dict type better by specifying key/value types
* tests: add missing db backend client files to `import_all_modules`