Tests on all supported OSes started failing with recent Python 3.13.0a5.

Tests succeeded on Linux and macOS with Python 3.13.0a4 and on Windows with Python 3.13.0a3.

Let's investigate the errors and determine on who's side fixes are needed. If urllib3's code has to be updated to work with Python 3.13, we have to check the 1.26.x branch too.

https://github.com/urllib3/urllib3/actions/runs/8317963362/job/22759351029