use async with in example client with encryption

examples/client-with-encryption.py

@@ -15,24 +15,19 @@ private_key = f"certificates/peer-private-key-example-{cert_idx}.pem"
 
 async def task(loop):
     url = "opc.tcp://0.0.0.0:4840/freeopcua/server/"
-    try:
-        client = Client(url=url)
-        await client.set_security(
-            SecurityPolicyBasic256Sha256,
-            certificate_path=cert,
-            private_key_path=private_key,
-            server_certificate_path="certificate-example.der"
-        )
-        await client.connect()
+    client = Client(url=url)
+    await client.set_security(
+        SecurityPolicyBasic256Sha256,
+        certificate_path=cert,
+        private_key_path=private_key,
+        server_certificate_path="certificate-example.der"
+    )
+    async with client:
         objects = client.nodes.objects
         child = await objects.get_child(['0:MyObject', '0:MyVariable'])
         print(await child.get_value())
         await child.set_value(42)
         print(await child.get_value())
-    except Exception:
-        _logger.exception('error')
-    finally:
-        await client.disconnect()
 
 
 def main():