Fix loading of private symbols within cimpl

Author: jgiannuzzi

src/confluent_kafka/cimpl/__init__.py

@@ -1,10 +1,18 @@
-try:
-    from .sasl2_3.cimpl import *
-    variant = "sasl2_3"
-except ImportError:
+import importlib
+
+variant = None
+
+for module_name in ("sasl2_3", "sasl2_2", "nodeps"):
     try:
-        from .sasl2_2.cimpl import *
-        variant = "sasl2_2"
-    except ImportError:
-        from .nodeps.cimpl import *
-        variant = "nodeps"
+        module = importlib.import_module(f".{module_name}.cimpl", __package__)
+        variant = module_name
+        for name in dir(module):
+            if not name.startswith("__"):
+                globals()[name] = getattr(module, name)
+        break
+    except ModuleNotFoundError:
+        pass
+
+if variant is None:
+    msg = "No valid native Python extension found"
+    raise ImportError(msg)