This URL for this repository is no longer in use -- use https://github.com/openedx/xblock-sdk instead (in the openedx org).
For more information about this move and why you're seeing this instead of being redirected, see: https://discuss.openedx.org/t/please-update-your-git-urls-for-edx-platform-and-several-other-repos/12387