Dependencies: Add greenlet package to test dependency group (#161)

All async tests apparently need it. Otherwise:

```shell
pytest -vvv -k test_async_pgvector
```
```python
ValueError: the greenlet library is required to use this function. No module named 'greenlet'
```

Author: amotl

poetry.lock

@@ -29,6 +29,7 @@ jupyterlab = "^3.6.1"
 optional = true
 
 [tool.poetry.group.test.dependencies]
+greenlet = "<4"
 pytest = "^7.4.3"
 pytest-asyncio = "^0.23.2"
 pytest-socket = "^0.7.0"