Container.started should return the same type as self
The type signature for `behave_utils.docker.Container.started()` should be:
```python
T = TypeVar("T")
...
@contextmanager
def started(self: T) -> Iterator[T]: ...
```
This affects the following classes:
- [x] `behave_utils.docker.Container`
- [x] `behave_utils.mysql.Mysql`
issue