Fix a security issue when retrieving a SSL certificate

Fix a security issue when retrieving a SSL certificate. See https://github.com/nuxeo/nuxeo-drive/security/code-scanning/1 for details.

We should use the default value set in ssl.SSLContext to prevent future similar issues:

Changed in version 3.6: The context is created with secure default values. The options OP_NO_COMPRESSION, OP_CIPHER_SERVER_PREFERENCE, OP_SINGLE_DH_USE, OP_SINGLE_ECDH_USE, OP_NO_SSLv2 (except for PROTOCOL_SSLv2), and OP_NO_SSLv3 (except for PROTOCOL_SSLv3) are set by default. The initial cipher suite list contains only HIGH ciphers, no NULL ciphers and no MD5 ciphers (except for PROTOCOL_SSLv2).