Python requests "certificate verify failed"
It turned out that during the server upgrade mentioned in the question an incorrectly-signed certificate was installed. HTTPS in the browser worked because of the root certificate differences between the Windows browser machine and Ubuntu Python client. HTTPS via a browser from the same Ubuntu machine on which Python was run revealed the certificate problem details.
The IP change had little to do with the problem except to confuse things.
Promoting my comment to an answer as:
- this answered my question
- this question is getting enough traffic I'd like to share the knowledge.
You're using an ancient version of requests. You'll get a more helpful message if you upgrade to 2.0 and if your site has a certificate mismatch you may be able to fix it by specifying the system certificates which will be able to verify an intermediate certificate. You can also just have requests not verify your certificate as Andre suggested.