When running the attached script with the attached cert, Python 3.7 raises an exception (see https://paste.pound-python.org/show/VLr84Yn2Fnz6RSKEq3ui/). In Python 3.6, the certificate is being accepted.

I don't see anything wrong with the self-signed certificate.

You can (hopefully) reproduce this by running minimal_server.py

The exception message is:

ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: IP address mismatch, certificate is not valid for '127.0.0.1'. (_ssl.c:1045)

The certificate is not valid for the URL. You are connection to a server by IP address, but the certificate is not valid for that IP address.

Hi.

I don't see why the certificate would not be valid for that address. Python 3.6 also accepts it without any modifications to the script. 

Output of openssl x509 -in cert.pem -noout -text

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            bc:28:67:9a:b0:fe:d6:b8
    Signature Algorithm: ecdsa-with-SHA256
        Issuer: CN = 127.0.0.1
        Validity
            Not Before: Mar 23 16:52:01 2017 GMT
            Not After : Mar 21 16:52:01 2027 GMT
        Subject: CN = 127.0.0.1
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:9d:e3:f2:f6:e2:8c:f3:25:82:3e:9e:bc:c5:69:
                    27:34:be:45:89:4a:51:ce:67:4e:b8:a0:b1:a2:bd:
                    fa:39:f9:38:85:a3:9c:a6:c4:c9:78:24:c7:17:5c:
                    2b:00:af:7f:73:e2:49:68:9c:37:29:ae:69:bf:b5:
                    49:06:a8:b8:1d
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                17:66:86:40:B1:C4:BF:77:09:C7:DC:9F:4D:78:4A:BF:07:19:AD:8C
            X509v3 Authority Key Identifier: 
                keyid:17:66:86:40:B1:C4:BF:77:09:C7:DC:9F:4D:78:4A:BF:07:19:AD:8C

            X509v3 Basic Constraints: 
                CA:TRUE
    Signature Algorithm: ecdsa-with-SHA256
         30:44:02:20:09:d2:c1:85:f9:c5:7f:78:3e:cc:90:78:25:dc:
         9e:76:ef:62:7a:e5:38:0a:a1:6c:c6:27:af:ed:ec:1d:12:06:
         02:20:5d:d0:de:8e:46:ee:e3:67:35:66:fe:11:6e:56:b5:70:
         72:16:33:92:66:0f:6c:da:51:0c:74:d8:c1:b8:8f:b5

Python 3.6 is a little more forgiving than Python 3.7. Python 3.7 uses OpenSSL's hostname verification algorithms, which interpret the RFCs more strictly. You have to include a SAN field of type IP address. Matching against CN has been deprecated for more than 15 years, see https://bugs.chromium.org/p/chromium/issues/detail?id=308330

Cheers!