Seems that in some conditions, OpenSSL will return ``SSL_ERROR_SYSCALL``
instead of ``SSL_ERROR_SSL`` when a certification verification has failed,
but the error parameters will still contain ``ERR_LIB_SSL`` and
``SSL_R_CERTIFICATE_VERIFY_FAILED``. We are now detecting this situation and
raising the appropiate ``ssl.SSLCertVerificationError``. Patch by Pablo
Galindo