api: Fetch server_settings when initializing Client.

We store the information about the version of the server and the feature
level for transparent compatibility handling.

zulip/zulip/__init__.py

@@ -486,6 +486,11 @@ class Client:
 
         self.has_connected = False
 
+        server_settings = self.get_server_settings()
+        self.zulip_version: Optional[str] = server_settings.get("zulip_version")
+        self.feature_level: int = server_settings.get("zulip_feature_level", 0)
+        assert self.zulip_version is not None
+
     def ensure_session(self) -> None:
 
         # Check if the session has been created already, and return

zulip_botserver/tests/test_server.py

@@ -25,6 +25,13 @@ class BotServerTests(BotServerTestCase):
         def handler_class(self) -> Any:
             return BotServerTests.MockMessageHandler()
 
+    def setUp(self) -> None:
+        # Since initializing Client invokes `get_server_settings` that fails in the test
+        # environment, we need to mock it to pretend that there exists a backend.
+        super().setUp()
+        self.patch = mock.patch("zulip.Client.get_server_settings", return_value=mock.Mock())
+        self.patch.start()
+
     def test_successful_request(self) -> None:
         available_bots = ["helloworld"]
         bots_config = {