Don't recheck version if api_versions data is already cached (#1738)

I noticed during local testing that version probing was happening twice when connecting to newer broker versions. This was because we call check_version() once explicitly, and then again implicitly within get_api_versions(). But once we have _api_versions data cached, we can just return it and avoid probing versions a second time.

kafka/conn.py

@@ -912,6 +912,9 @@ def _handle_api_version_response(self, response):
         return self._api_versions
 
     def get_api_versions(self):
+        if self._api_versions is not None:
+            return self._api_versions
+
         version = self.check_version()
         if version < (0, 10, 0):
             raise Errors.UnsupportedVersionError(