Loss of the cookie state when using max-age=0.

[luckydenis]

Summary.
Loss of the cookie state when receiving a response from the server containing the lifetime in the format max-age=0.

Expected Result

import requests
r = requests.get('http://127.0.0.1:5000/expire')
>>> r.headers
{'Set-Cookie': 'test=test; Path=/; Max-Age=0', 'Content-Type': 'text/html; charset=utf-8', 'Content-Length': '10', 'Server': 'Werkzeug/1.0.1 Python/3.8.7', 'Date': 'Fri, 05 Feb 2021 09:26:48 GMT'}
>>> r.cookies
<RequestsCookieJar[Cookie(version=0, name='test', value='test', port=None, port_specified=False, domain='127.0.0.1', domain_specified=False, domain_initial_dot=False, path='/', path_specified=True, secure=False, expires=1612520808, discard=False, comment=None, comment_url=None, rest={}, rfc2109=False)]>

Actual Result

>>> import requests
>>> r = requests.get('http://127.0.0.1:5000/expire')
>>> r.headers
{'Set-Cookie': 'test=test; Path=/; Max-Age=0', 'Content-Type': 'text/html; charset=utf-8', 'Content-Length': '14', 'Server': 'Werkzeug/1.0.1 Python/3.8.7', 'Date': 'Fri, 05 Feb 2021 09:16:43 GMT'}
>>> r.cookies
<RequestsCookieJar[]>

Reproduction Steps

app.py

from flask import Flask, make_response


app = Flask(__name__)


@app.route('/expire')
def expire():
    return "Expired cookie", {'Set-Cookie': 'test=test; Path=/; Max-Age=0'}

Start app: FLASK_APP=app.py flask run -p 5000

test.py

import requests


r = requests.get('http://127.0.0.1:5000/expire')
print('headers:', r.headers)
print()
print('cookies:', r.cookies)

output

headers: {'Set-Cookie': 'test=test; Path=/; Max-Age=0', 'Content-Type': 'text/html; charset=utf-8', 'Content-Length': '14', 'Server': 'Werkzeug/1.0.1 Python/3.8.7', 'Date': 'Fri, 05 Feb 2021 09:40:32 GMT'}

cookies: <RequestsCookieJar[]>

System Information

$ python -m requests.help

{
  "chardet": {
    "version": "4.0.0"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "2.10"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.8.7"
  },
  "platform": {
    "release": "5.8.0-41-generic",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.25.1"
  },
  "system_ssl": {
    "version": "1010106f"
  },
  "urllib3": {
    "version": "1.26.3"
  },
  "using_pyopenssl": false
}

This command is only available on Requests v2.16.4 and greater. Otherwise,
please provide some basic information about your system (Python version,
operating system, &c).

[luckydenis]

Good evening.
The cookie state is deleted at this location.
https://github.com/python/cpython/blob/39aeb9ff9064808b08ec629403edbc36a232369b/Lib/http/cookiejar.py#L1548-L1561