こちらと同じことを Rest で行います。
Python3 で Cloud Firestore のデータを作成 (Create)

firestore_create_rest.py

# ! /usr/bin/python
# -*- coding: utf-8 -*-
#
#	firestore_create_rest.py
#
#					Jan/31/2020
#
# ------------------------------------------------------------------
import  sys
import  json
import  requests
#
from get_token import get_token_proc
# ------------------------------------------------------------------
def add_data_proc(url_base,token,key,name,population,date_mod):
	unit_aa = {}
	unit_aa["name"] = {"stringValue": name}
	unit_aa["population"] = {"integerValue": population}
	unit_aa["date_mod"] = {"timestampValue": date_mod}
	payload = {"fields": unit_aa}
	str_json = json.dumps(payload)
	headers = {"Authorization": "Bearer " + token,
		"Content-Type": 'application/json'}
	url = url_base + "?documentId=" + key
	try:
		rr=requests.post(url,data=str_json,headers=headers)
		print(rr.text)
	except Exception as ee:
		sys.stderr.write("*** error *** in requests.post ***\n")
		sys.stderr.write(str(ee) + "\n")
# ------------------------------------------------------------------
sys.stderr.write("*** 開始 ***\n")
#
token = get_token_proc()
#
# print(token)
#
url="https://firestore.googleapis.com/v1/projects/project-jan25-2020/databases/(default)/documents/cities"
#
add_data_proc(url,token,'t0921','宇都宮',38921,"2010-10-5T00:15:07.0Z")
add_data_proc(url,token,'t0922','小山',56417,"2010-9-12T01:16:07.0Z")
add_data_proc(url,token,'t0923','佐野',92185,"2010-12-8T02:17:07.0Z")
add_data_proc(url,token,'t0924','足利',39546,"2010-7-4T03:18:07.0Z")
add_data_proc(url,token,'t0925','日光',18492,"2010-10-2T04:19:07.0Z")
#
sys.stderr.write("*** 終了 ***\n")
# ------------------------------------------------------------------

get_token.py

# -*- coding: utf-8 -*-
#
#	firestore_token.py
#
#					Jan/31/2020
#
# ------------------------------------------------------------------
import  os
import  sys
import	subprocess
# ------------------------------------------------------------------
def get_token_proc():
	token = ""
	command=["gcloud","auth","application-default","print-access-token"]
#
	try:
		my_env = os.environ.copy()
		token_byte = subprocess.check_output(command,env=my_env)
		token = token_byte.decode('utf-8')
	except Exception as ee:
		sys.stderr.write("*** error *** in subprocess.check_output ***\n")
		sys.stderr.write(str(ee) + "\n")
#
	return	token[:-1]
# ------------------------------------------------------------------

実行コマンド

export GOOGLE_APPLICATION_CREDENTIALS="***.json"
./firestore_create_rest.py