Firestore client in python (as user) using firebase_admin or google.cloud.firestore

I am building a python client-side application that uses Firestore. I have successfully used Google Identity Platform to sign up and sign in to the Firebase project, and created a working Firestore client using google.cloud.firestore.Client which is authenticated as a user:

import json
import requests
import google.oauth2.credentials
from google.cloud import firestore

request_url = f"https://identitytoolkit.googleapis.com/v1/accounts:signInWithPassword?key={self.__api_key}"
headers = {"Content-Type": "application/json; charset=UTF-8"}
data = json.dumps({"email": self.__email, "password": self.__password, "returnSecureToken": True})
response = requests.post(request_url, headers=headers, data=data)
try:
    response.raise_for_status()
except (HTTPError, Exception):
    content = response.json()
    error = f"error: {content['error']['message']}"
    raise AuthError(error)

json_response = response.json()
self.__token = json_response["idToken"]
self.__refresh_token = json_response["refreshToken"]

credentials = google.oauth2.credentials.Credentials(self.__token,
                                                    self.__refresh_token,
                                                    client_id="",
                                                    client_secret="",
                                                    token_uri=f"https://securetoken.googleapis.com/v1/token?key={self.__api_key}"
                                                    )

self.__db = firestore.Client(self.__project_id, credentials)

I have the problem, however, that when the token has expired, I get the following error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/google/api_core/grpc_helpers.py", line 57, in error_remapped_callable
    return callable_(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/grpc/_channel.py", line 826, in __call__
    return _end_unary_response_blocking(state, call, False, None)
  File "/usr/local/lib/python3.7/dist-packages/grpc/_channel.py", line 729, in _end_unary_response_blocking
    raise _InactiveRpcError(state)
grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
    status = StatusCode.UNAUTHENTICATED
    details = "Missing or invalid authentication."
    debug_error_string = "{"created":"@1613043524.699081937","description":"Error received from peer ipv4:172.217.16.74:443","file":"src/core/lib/surface/call.cc","file_line":1055,"grpc_message":"Missing or invalid authentication.","grpc_status":16}"
>

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

Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/home/my_app/src/controllers/im_alive.py", line 20, in run
    self.__device_api.set_last_updated(utils.device_id())
  File "/home/my_app/src/api/firestore/firestore_device_api.py", line 21, in set_last_updated
    "lastUpdatedTime": self.__firestore.SERVER_TIMESTAMP
  File "/home/my_app/src/api/firestore/firestore.py", line 100, in update
    ref.update(data)
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/firestore_v1/document.py", line 382, in update
    write_results = batch.commit()
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/firestore_v1/batch.py", line 147, in commit
    metadata=self._client._rpc_metadata,
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/firestore_v1/gapic/firestore_client.py", line 1121, in commit
    request, retry=retry, timeout=timeout, metadata=metadata
  File "/usr/local/lib/python3.7/dist-packages/google/api_core/gapic_v1/method.py", line 145, in __call__
    return wrapped_func(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/google/api_core/retry.py", line 286, in retry_wrapped_func
    on_error=on_error,
  File "/usr/local/lib/python3.7/dist-packages/google/api_core/retry.py", line 184, in retry_target
    return target()
  File "/usr/local/lib/python3.7/dist-packages/google/api_core/timeout.py", line 214, in func_with_timeout
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/google/api_core/grpc_helpers.py", line 59, in error_remapped_callable
    six.raise_from(exceptions.from_grpc_error(exc), exc)
  File "<string>", line 3, in raise_from
google.api_core.exceptions.Unauthenticated: 401 Missing or invalid authentication.

I have tried omitting the token and only specifying the refresh token, and then calling credentials.refresh(), but the expires_in in the response from the https://securetoken.googleapis.com/v1/token endpoint is a string instead of a number (docs here), which makes _parse_expiry(response_data) in google.oauth2._client.py:257 raise an exception.

Is there any way to use the firestore.Client from either google.cloud or firebase_admin and have it automatically handle refreshing tokens, or do I need to switch to the manually calling the Firestore RPC API and refreshing tokens at the correct time?

Note: There are no users interacting with the python app, so the solution must not require user interaction.

from Firestore client in python (as user) using firebase_admin or google.cloud.firestore