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

playlist = After entire playlist
after_video = After downloading all formats of a video

Adds option `--use-postprocessor` to enable them

* All modules and binary names are changed
* All documentation references changed
* yt-dlp no longer loads youtube-dlc config files
* All URLs changed to point to organization account

Co-authored-by: Pccode66
Co-authored-by: pukkandan

Extractor plugins are loaded from <root-dir>/ytdlp_plugins/extractor/__init__.py

Inspired by https://github.com/un-def/dl-plus

:ci skip dl