Hi,
This may be a tad bit too but late. But I encountered the same issue and another issue when I tried to follow the Xblock documentation that specified Python 2.7.
For those who may chance upon this, it may be best to read through the READ.rst file on the XBlock SDK’s Github account. For reference, see @braden’s response here: Xblock SDK: Setting entry points for virtenv - #2 by braden.