make block.get_parent() work.

Co-Authored-By: Christina Roberts <christina@edx.org>
Co-Authored-By: Daniel Friedman <dfriedman@edx.org>
Co-Authored-By: Don Mitchell <dmitchell@edx.org>
Merge pull request #6295 from edx/tim/edxnotes-get_parent

make block.get_parent() work.