How To Use Token In A Resumable Upload With Google Drive API (Python)

I'm trying to implement resumable upload with Google Drive API via Python. It's important it will be resumable because I need to upload a few GB every time.

So I followed this tutorial. Yet, I have no idea how to use my token inside headers.

The problem is in uploadFile() --> access_token=?

It can't be the token itself (or at least not in its usual form)

from __future__ import print_function
import pickle
import os.path
import json
import os
import requests
from googleapiclient.discovery import build
from google_auth_oauthlib.flow import InstalledAppFlow
from google.auth.transport.requests import Request
from googleapiclient.http import MediaFileUpload

def auth():
    creds = None
    if os.path.exists('token.pickle'):
        with open('token.pickle', 'rb') as token:
            creds = pickle.load(token)
    # 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)
            creds = flow.run_local_server(port=0)
        # Save the credentials for the next run
        with open('token.pickle', 'wb') as token:
            pickle.dump(creds, token)
    global service
    service = build('drive', 'v3', credentials=creds)

def uploadFile():
    access_token = ??? #As I get it, it should be a string. But how do I stringify a token?

    filename = './20200229_151839_008.jpg'
    filesize = os.path.getsize(filename)

    # 1. Retrieve session for resumable upload.
    headers = {"Authorization": "Bearer " + access_token, "Content-Type": "application/json"}
    params = {
        "name": "20200229_151839_008.jpg",
        "mimeType": "image/img"
    }
    r = requests.post(
        "https://www.googleapis.com/upload/drive/v3/files?uploadType=resumable",
        headers=headers,
        data=json.dumps(params)
    )
    location = r.headers['Location']


    # 2. Upload the file.
    headers = {"Content-Range": "bytes 0-" + str(filesize - 1) + "/" + str(filesize)}
    r = requests.put(
        location,
        headers=headers,
        data=open(filename, 'rb')
    )
    print(r.text)

Any help would be appreciated.

Thank you.