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
When the manifest is not downloadable by native downloader, it already is able to detect it and switch to `ffmpeg`. So there doesn't seem to be a reason anymore to use ffmpeg as the preferred downloader
