Fix #173: HTTPS request doesn't work when disable_ssl_certificate_validation=True

Sets `SSLContext.verify_mode` to `ssl.CERT_OPTIONAL` if `disable_ssl_certificate_validation` is `True` otherwise it is set to `ssl.CERT_REQUIRED`.
I'm pretty sure this causes Python 3.1 to always raise an exception, even when certification validation is explicitly not disabled, - i.e. when disable_ssl_certificate_validation is enabled.

fwiw, there is another pending patch for this issue; pull #269 , which has bitrotted.