Make client work with gevent monkey patching

Hello,

- OS type and version: Mac OS Big Sur ver 11.6.1
- Python version: Python 3.8.9
- pip version: pip 21.1.2
- `google-cloud-pubsublite` version: 1.4.1

#### Steps to reproduce

1. using tutorial codes : https://cloud.google.com/pubsub/lite/docs/publishing
2. add two more lines at the beginning:
```
from gevent import monkey
monkey.patch_all()
```

#### Code example

```python
from gevent import monkey
monkey.patch_all()

from google.cloud.pubsublite.cloudpubsub import PublisherClient
from google.cloud.pubsublite.types import (
CloudRegion,
CloudZone,
MessageMetadata,
TopicPath,
)

# TODO(developer):
# project_number = 1122334455
# cloud_region = "us-central1"
# zone_id = "a"
# topic_id = "your-topic-id"
# num_messages = 100
# regional = True

if regional:
location = CloudRegion(cloud_region)
else:
location = CloudZone(CloudRegion(cloud_region), zone_id)

topic_path = TopicPath(project_number, location, topic_id)

# PublisherClient() must be used in a `with` block or have __enter__() called before use.
with PublisherClient() as publisher_client:
for message in range(num_messages):
data = f"{message}"
# Messages of the same ordering key will always get published to the same partition.
# When ordering_key is unset, messsages can get published ot different partitions if
# more than one partition exists for the topic.
api_future = publisher_client.publish(
topic_path, data.encode("utf-8"), ordering_key="testing"
)
# result() blocks. To resolve api futures asynchronously, use add_done_callback().
message_id = api_future.result()
message_metadata = MessageMetadata.decode(message_id)
print(
f"Published {data} to partition {message_metadata.partition.value} and offset {message_metadata.cursor.offset}."
)

print(
f"Finished publishing {num_messages} messages with an ordering key to {str(topic_path)}."
)
```

When running the code with the monkey.patch_all(), publish() hangs.

I debugged a little bit, looks "thread" and "select" patch affect pubsublite. If I didn't patch these two as below, it worked as expected. Not sure if pubsublite is gevent friendly? My case is I need patch all.
```
from gevent import monkey
monkey.patch_all(thread=False, select=False)
```

Thanks!