OSError: Unable to communicate with the jupyter_dash.., when using a custom env

Running `JupyterDash.infer_jupyter_proxy_config()` throws an error, when using a kernel from the custom environment (not BBS-BBG). The following error is observed:

```
---------------------------------------------------------------------------
OSError Traceback (most recent call last)
in
1 time.sleep(3)
----> 2 JupyterDash.infer_jupyter_proxy_config()
3 cyto.load_extra_layouts()

~/test-bbs-bbg-notebook/lib/python3.8/site-packages/jupyter_dash/jupyter_app.py in infer_jupyter_proxy_config(cls)
71 else:
72 # Assume classic notebook or JupyterLab
---> 73 _request_jupyter_config()
74
75 def __init__(self, name=None, server_url=None, **kwargs):

~/test-bbs-bbg-notebook/lib/python3.8/site-packages/jupyter_dash/comms.py in _request_jupyter_config(timeout)
60 if (time.time() - t0) > timeout:
61 # give up
---> 62 raise EnvironmentError(
63 "Unable to communicate with the jupyter_dash notebook or JupyterLab \n"
64 "extension required to infer Jupyter configuration."

OSError: Unable to communicate with the jupyter_dash notebook or JupyterLab
extension required to infer Jupyter configuration.
```

Start by reproducing the reported call to JupyterDash.infer_jupyter_proxy_config() with a kernel from the custom environment. Trace the paths shown in jupyter_dash/jupyter_app.py and jupyter_dash/comms.py; done means the call no longer raises the reported communication OSError.

由索引模型根据 Issue 内容生成。