It seems like ssl module is not working for me:

(virtualenv) user@host:~$ python
Python 3.6.0 (v3.6.0:41df79263a11, Dec 22 2016, 17:23:13) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ssl
>>> import socket
>>> context = ssl.create_default_context()
>>> conn = context.wrap_socket(socket.socket(socket.AF_INET), server_hostname="www.python.org")
>>> conn.connect(("www.python.org", 443))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ssl.py", line 1093, in connect
    self._real_connect(addr, False)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ssl.py", line 1084, in _real_connect
    self.do_handshake()
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ssl.py", line 1061, in do_handshake
    self._sslobj.do_handshake()
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ssl.py", line 683, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:749)
>>> 

It seems like a problem with OpenSSL in Sierra itself. Because the following command does not work either:

(virtualenv) user@host:~$ openssl s_client -connect www.python.org:443
CONNECTED(00000003)
depth=1 /C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
 0 s:/businessCategory=Private Organization/1.3.6.1.4.1.311.60.2.1.3=US/1.3.6.1.4.1.311.60.2.1.2=Delaware/serialNumber=3359300/street=16 Allen Rd/postalCode=03894-4801/C=US/ST=NH/L=Wolfeboro/O=Python Software Foundation/CN=www.python.org
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
 1 s:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA
---
Server certificate
-----BEGIN CERTIFICATE-----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ejAkBggrBgEFBQcwAYYYaHR0cDovL29jc3AuZGlnaWNlcnQuY29tMFIGCCsGAQUF
BzAChkZodHRwOi8vY2FjZXJ0cy5kaWdpY2VydC5jb20vRGlnaUNlcnRTSEEyRXh0
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zQ7IDdwQAAABVkgj4a8AAAQDAEcwRQIhAMhsxamO6hrRjfNmH4Yj/cnJo72cmTHm
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mPgfZIK+OozuuSgdTPxxUV1nk9RE0QpnrLtPT/vEAAABVkgj4XYAAAQDAEcwRQIg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-----END CERTIFICATE-----
subject=/businessCategory=Private Organization/1.3.6.1.4.1.311.60.2.1.3=US/1.3.6.1.4.1.311.60.2.1.2=Delaware/serialNumber=3359300/street=16 Allen Rd/postalCode=03894-4801/C=US/ST=NH/L=Wolfeboro/O=Python Software Foundation/CN=www.python.org
issuer=/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
---
No client certificate CA names sent
---
SSL handshake has read 3524 bytes and written 456 bytes
---
New, TLSv1/SSLv3, Cipher is AES128-SHA
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
    Protocol  : TLSv1
    Cipher    : AES128-SHA
    Session-ID: 239A88888D772216EDDD8C204996A901BAD27D50B929E6A626955472456077B6
    Session-ID-ctx: 
    Master-Key: FB1EAC13FFD2F42DA5BF0364E3A08A869DD129389374416B6574F6D852692888F991B79027143A81963FA7594FFB85BD
    Key-Arg   : None
    Start Time: 1482618121
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)
---

Connecting to `www.python.org` (or any other secure site) works on web browsers. Also, `DigiCert High Assurance EV Root CA` exists in the System Roots section of Keychain.

I guess the problem might be that ssl module does not use the Keychain, like `openssl` command.

Regards

From the output you supplied, you appear to be using Python 3.6.0 from the python.org macOS binary installer.  Please read the ReadMe.rtf file for the installer which should have been displayed when you installed Python 3.6; a copy of it is also installed in /Applications/Python 3.6.  You can read it by double-clicking on the icon in the Finder or by entering the following shell command:

open "/Applications/Python 3.6/ReadMe.rtf"

In it, the section on "Certificate verification and OpenSSL" notes that, as of the 3.6.0 installer, the Python supplied no longer links with the deprecated Apple-supplied system OpenSSL libraries but rather supplies a private copy of OpenSSL 1.0.2 which does not automatically access the system default root certificates.  "For 3.6.0, a sample command script is included in /Applications/Python 3.6 to install a curated bundle of default root certificates from the third-party certifi package (https://pypi.python.org/pypi/certifi).  If you choose to use certifi, you should consider subscribing to the project's email update service to be notified when the certificate bundle is updated."  You can run the command script by double-clicking on it or by entering the shell command:

open "/Applications/Python 3.6/Install Certificates.command"

If necessary, you can adapt the script for other options rather than using the certifi-supplied bundle.  For later releases, other standard options will likely be provided.

I don't understand your comment that using the system openssl command doesn't work, e.g. "openssl s_client -connect www.python.org:443".  The output you supply appears to show it working as expected.  But, in any case, that's not relevant to the Python 3.6 usage as different OpenSSL versions and libraries are being used.

Also note that this description only applies to the Python 3.6 supplied by the python.org macOS installer.  mscOS Pythons supplied by third-party distributors likely link with each distributor's version of OpenSSL and follow their root certificate policies.

I've also added a note to the release downloads "Notes on this release" section:

"macOS users: If you are using the Python 3.6 from the python.org binary installer linked on this page, please carefully read the Important Information displayed during installation; this information is also available after installation by clicking on /Applications/Python 3.6/ReadMe.rtf. There is important information there about changes in the 3.6.0 installer-supplied Python, particularly with regard to SSL certificate validation."