I'm posting this as a PSA, because I just wasted a few hours (should have read the release notes in full) trying to resolve an "SSL: CERTIFICATE_VERIFY_FAILED" exception every time I requested a URL over HTTPS. All my Googling take me to promising-but-irrelevant StackOverflow threads. It turns out that on OSX Python 3.6 no longer uses the systems trusted certificates, so after install it has no certificates at all (except a hardcoded one for pip) and can't validate any SSL connections.
To fix this you need to run a command [0] included in the installer that will download and install the certifi package, which is a collection of trusted certificates. If I understand the release notes correctly you are also now responsible for making sure your certificates are up to date - the release notes recommend subscribing to the mailing list.
Personally, I think this sucks. It makes Python way less friendly to new users, and I don't want to be on the hook for keeping my certificates up-to-date. At the very least, the installer should run the post-install script automatically, so that SSL connections just work.
The ReadMe with more info lives at /Applications/Python\ 3.6/ReadMe.rtf
[0] /Applications/Python\ 3.6/Install\ Certificates.command
https://lwn.net/Articles/711954/
This will involve Python ceasing to use OpenSSL in favour of native SSL APIs (except on the Unix platforms where OpenSSL is the native API).
Right now your Python uses OpenSSL, but nobody at all was responsible for keeping the trust store up-to-date. So, making you responsible was in a sense a step up. Certifi basically bundles the Mozilla NSS trust store, which is also used by the BSDs, Linux distros, Haiku and whatever else isn't Apple or Microsoft. But it won't reflect OS X changes, for better or worse. On the other hand, if that PEP goes ahead and Python trusts the OS X built-in stores, Python code may work on your Mac, but not on some guy's FreeBSD machine, because the different trust stores don't agree about whether to trust a certificate. So, swings and roundabouts.