Install chrome native client (nacl)

I am trying to install chrome native client.
I went to this page to download and install the sdk. I followed the instructions in this page however when I tried to execute the naclsdk list commnad or any naclsdk command I got an error –

third_party.fancy_urllib.InvalidCertificateException: Host returned an invalid certificate ([SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)):
To learn more, see
Usage: [options]

what is the problem here? ,is there a problem with a certificate?