* Move Brightcove embed extraction and tests into the IEs
* Split `BrightcoveNewBaseIE` from `BrightcoveNewIE`
* Fix bug in ade1fa70cb with the "wrong" spelling of `referrer` being smuggled
Closes#5539
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