fix(ext.autodoc): fix logging of tracebacks

`traceback.format_exception()` returns a list of strings containing already newlines.
Sphinx just logs the repr of that list, which makes it impossible to read tracebacks easily.

the real and helpful exception trace would be:
```
Traceback (most recent call last):
   File "/usr/local/lib/python3.11/dist-packages/sphinx/ext/autodoc/importer.py", line 175, in import_module
    module = importlib.import_module(modname)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/lib/python3.11/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "<frozen importlib._bootstrap>", line 1206, in _gcd_import
   File "<frozen importlib._bootstrap>", line 1178, in _find_and_load
   File "<frozen importlib._bootstrap>", line 1149, in _find_and_load_unlocked
   File "<frozen importlib._bootstrap>", line 690, in _load_unlocked
   File "<frozen importlib._bootstrap_external>", line 940, in exec_module
   File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
   File "/root/module.py", line 58, in <module>
    ...
 AttributeError: 'NoneType' object has no attribute 'lower'

The above exception was the direct cause of the following exception:

 Traceback (most recent call last):
   File "/usr/local/lib/python3.11/dist-packages/sphinx/ext/autodoc/importer.py", line 269, in import_object
    module = import_module(modname, try_reload=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/dist-packages/sphinx/ext/autodoc/importer.py", line 187, in import_module
    raise ImportError(exc, traceback.format_exc()) from exc
 ImportError: (AttributeError("'NoneType' object has no attribute 'lower'"),
```

Author: spaceone

sphinx/ext/autodoc/importer.py

@@ -281,7 +281,7 @@ def _import_from_module_and_path(
             # import_module() raises ImportError having real exception obj and
             # traceback
             real_exc = exc.args[0]
-            traceback_msg = traceback.format_exception(exc)
+            traceback_msg = ''.join(traceback.format_exception(exc))
             if isinstance(real_exc, SystemExit):
                 err_parts.append(
                     'the module executes module level statement '