Authorizing Google API on Headless Machine in Python

Running Linux (ubuntu and python in docker on Raspberry OS) on a headless Raspberry Pi 4.

Followed Google's Python Quickstart Guide and got it to the point where it asks me to go to a URL like this:

Please visit this URL to authorize this application: https://accounts.google.com/o/oauth2/auth?response_type=code&client_id=xxxxx&redirect_uri=http%3A%2F%2Flocalhost%3A59875%2F&scope=https%3A%2F%2Fwww.googleapis.com%2Fauth%2Fcalendar.readonly&state=xxxxx&access_type=offline

Because I am headless and on SSH, I copied to my GUI computer and sign in authorize, but I get a request back to localhost:59875/?state=xxxxx&code=xxxxx&scope=xxxx which obviously does not open as nothing on my local machine is listening. I see that redirect URI in the above link with localhost, but changing that is invalid per this.

How can I authorize Google API on a headless machine?

Short of creating a webpage on my pi and using that to run the python (which is more than I want to do right now, especially not knowing if it will work), how can I do this?

EDIT
I am creating a python program that will interact with Google Calendar. I have a developer account and credentials, and a personal account and credentials. I am trying to create this under the developer credentials and then enable (long term) users to authenticate themselves. Hence the test users.

To avoid redirection after successful Google OAuth2 authorization, and instead fetch the authorization code that you manually paste into the script you can use the special value urn:ietf:wg:oauth:2.0:oob defined by the OAuth2 specification.

Taking the calendar quickstart code and modifying it, it can be used something like this:

...
# If there are no (valid) credentials available, let the user log in.
    if not creds or not creds.valid:
        if creds and creds.expired and creds.refresh_token:
            creds.refresh(Request())
        else:
            flow = InstalledAppFlow.from_client_secrets_file(
                'credentials.json', SCOPES,
                redirect_uri='urn:ietf:wg:oauth:2.0:oob')

            auth_uri, _ = flow.authorization_url()
            print(f'Please visit {auth_uri} on your local computer')

            # The user will get an authorization code. This code is used to get the
            # access token.
            code = input('Enter the authorization code: ')
            flow.fetch_token(code=code)

            creds = flow.credentials
        # Save the credentials for the next run
        with open('token.json', 'w') as token:
            token.write(creds.to_json())
...

Note There is more than one way to achieve this: you can create a service account to be used by the app, and share your calendar with the service app user. You would then access the end user's calendar by using the calendarId param.

You should explore the Google API documentation and see what fits you the best.