First, to solve this specific error, it seems you need to upgrade urllib3. (Or, potentially, requests, which can include its own copy of urllib3 – you’re currently using an independent urllib3, though.)
What OS are you on? If you’re lucky, there might be an update available. If there isn’t, it may be best to use certbot-auto (which installs Python dependencies separately in a virtual environment) or upgrade to a newer release of your OS, which might happen to package a newer requests or urllib3.
Second, this error may or may not be important: something went wrong, then this bug is in the urllib3 error handling. Fixing it may just reveal what the original error is and leave you to solve that. Instead, if you can somehow figure out what network problem is happening, you might be able to resolve or work around it and ignore the error handling bug.
(Edits: Expanded the post.)