[cleanup] Upgrade syntax

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

ytdlp_plugins/extractor/sample.py

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 # ⚠ Don't use relative imports
 from yt_dlp.extractor.common import InfoExtractor
 

ytdlp_plugins/postprocessor/sample.py

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 # ⚠ Don't use relative imports
 from yt_dlp.postprocessor.common import PostProcessor