After porting `stx-nova-api-proxy` to Debian, I noticed that this error started to appear in other subcommands that worked normally in the CentOS based version (e.g. `nova flavor create`).

Upon further investigation, I found out that the code was breaking due to some request headers that were being passed to the Python 3's built-in `http` library with `NoneType` as their values (where, in fact, they should've been passed as strings [or not passed at all]):

...
'HTTP_OPENSTACK_SYSTEM_SCOPE': None,
'HTTP_X_DOMAIN_ID': None,
'HTTP_X_DOMAIN_NAME': None
...

In Python 2.7 – and therefore in the CentOS-based version of `nova-api-proxy` – this wasn't a problem because the builtin `httplib` (equivalent of Python 3's `http`) library handled the conversion to string (even of `NoneType`):

https://github.com/enthought/Python-2.7.3/blob/master/Lib/httplib.py#L938

However, in Python 3 – and therefore in the Debian-based version of `nova-api-proxy` – an exception is thrown instead:

https://github.com/python/cpython/blob/044fb4fb53594b37de8188cb36f3ba33ce2d617e/Lib/http/client.py#L1262

Therefore, some adjustments related to the Python 2.7 to Python 3 migration need to be made to the `nova-api-proxy` source code, something I will do in this LP.

----

Unfortunately, the original exception thrown during pause/unpause/suspend/resume is still occurring. The debug logs in `nova-api-proxy` shows the following tracestack:

```
2023-01-31 11:27:50,871.871 6 DEBUG nova_api_proxy.common.service [-] Traceback (most recent call last):
  File "/var/lib/openstack/lib/python3.9/site-packages/eventlet/wsgi.py", line 573, in handle_one_response
    result = self.application(self.environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 143, in __call__
    return resp(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/routes/middleware.py", line 153, in __call__
    response = self.app(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 143, in __call__
    return resp(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 129, in __call__
    resp = self.call_func(req, *args, **kw)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 193, in call_func
    return self.func(req, *args, **kwargs)
  File "/var/lib/openstack/lib/python3.9/site-packages/keystonemiddleware/auth_token/__init__.py", line 341, in __call__
    response = req.get_response(self._app)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/request.py", line 1313, in send
    status, headers, app_iter = self.call_application(
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/request.py", line 1278, in call_application
    app_iter = application(self.environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 143, in __call__
    return resp(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/routes/middleware.py", line 153, in __call__
    response = self.app(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 143, in __call__
    return resp(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 143, in __call__
    return resp(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/webob/dec.py", line 143, in __call__
    return resp(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/nova_api_proxy/apps/proxy.py", line 50, in __call__
    result = self.proxy_app(environ, start_response)
  File "/var/lib/openstack/lib/python3.9/site-packages/paste/proxy.py", line 236, in __call__
    res = conn.getresponse()
  File "/usr/lib/python3.9/http/client.py", line 1347, in getresponse
    response.begin()
  File "/usr/lib/python3.9/http/client.py", line 307, in begin
    version, status, reason = self._read_status()
  File "/usr/lib/python3.9/http/client.py", line 276, in _read_status
    raise RemoteDisconnected("Remote end closed connection without"
http.client.RemoteDisconnected: Remote end closed connection without response write /var/lib/openstack/lib/python3.9/site-packages/nova_api_proxy/common/service.py:91
```

Right now, I'm trying to understand what might be leading to this error.

As this behavior already happened when `nova-api-proxy` was still based on CentOS, I'm tending to believe that this is more related to `python-novaclient` itself.