`Brightcove` is difficult to migrate because it's subclasses may depend
on the signature of the current functions. So it is left as-is for now
Note: Tests have not been migrated
Using https://github.com/asottile/pyupgrade
1. `__future__` imports and `coding: utf-8` were removed
2. Files were rewritten with `pyupgrade --py36-plus --keep-percent-format`
3. f-strings were cherry-picked from `pyupgrade --py36-plus`
Extractors are left untouched (except removing header) to avoid unnecessary merge conflicts
