Additional fix to initialise connection.open to False

Additional fix to define and initialise the `connection.open` var to False earlier, to prevent the following error:
```
Traceback (most recent call last):
  File "/Users/sander.goos/opt/anaconda3/envs/pysql-2.0.2/lib/python3.9/site-packages/databricks/sql/client.py", line 124, in __del__
    if self.open:
AttributeError: 'Connection' object has no attribute 'open'
```

Integration tests.

Author: sander-goos

cmdexec/clients/python/src/databricks/sql/client.py

@@ -83,6 +83,7 @@ def __init__(self,
         # Databricks runtime will return native Arrow types for timestamps instead of Arrow strings
         # (True by default)
 
+        self.open = False
         self.host = server_hostname
         self.port = kwargs.get("_port", 443)
         self.disable_pandas = kwargs.get("_disable_pandas", False)