[TSD] fixes toggle annotation name

Addresses the [problem](https://github.com/edx/edx-val/pull/268/#issuecomment-767892293) with the waffle flag naming mentioned in https://github.com/edx/edx-val/pull/268.

Also, resolves the documentation problem for [`edxval.override_existing_imported_transcripts`](https://docs.google.com/spreadsheets/d/1xWbEL6oNu6D84WKBs3aViLRM40xk-vmnmmGAeLyR09A/edit?ts=6008a109&pli=1#gid=0&range=A178) in the doc-athon.

Also removes the python 3.5 function that was added because the repository was supported python 3.5 at that time.