Fix bug from when .split() was used

Author: haard

src/pythonjsonlogger/jsonlogger.py

@@ -134,7 +134,7 @@ def _str_to_fn(self, fn_as_str):
             return fn_as_str
 
         path, _, function = fn_as_str.rpartition('.')
-        module = importlib.import_module('.'.join(path))
+        module = importlib.import_module(path)
         return getattr(module, function)
 
     def parse(self):