TopicCollection is missing in __all__

Importing TopicCollection as
```
from confluent_kafka import TopicCollection
```

raises:
```
error: Module "confluent_kafka" has no attribute "TopicCollection"
```

The type is not listed in `__all__` at
https://github.com/confluentinc/confluent-kafka-python/blob/5a87879681d28375a55203c4839338b13b668046/src/confluent_kafka/__init__.py#L25