fix: remove decode() on error_message string

The integration tests failed because a test called 'decode()' on a
string-type variable - the GitLabException class handles byte-to-string
conversion already in its __init__. This commit removes the call to
'decode()' in the test.

```
Traceback (most recent call last):
  File "./tools/python_test_v4.py", line 801, in <module>
    assert 'Retry later' in error_message.decode()
AttributeError: 'str' object has no attribute 'decode'
```

tools/python_test_v4.py

@@ -798,7 +798,7 @@
     except gitlab.GitlabCreateError as e:
         error_message = e.error_message
         break
-assert 'Retry later' in error_message.decode()
+assert 'Retry later' in error_message
 [current_project.delete() for current_project in projects]
 settings.throttle_authenticated_api_enabled = False
 settings.save()