[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

yt_dlp/postprocessor/embedthumbnail.py

@@ -1,6 +1,3 @@
-# coding: utf-8
-from __future__ import unicode_literals
-
 import base64
 import imghdr
 import os
@@ -61,7 +58,7 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
         return int(mobj.group('w')), int(mobj.group('h'))
 
     def _report_run(self, exe, filename):
-        self.to_screen('%s: Adding thumbnail to "%s"' % (exe, filename))
+        self.to_screen(f'{exe}: Adding thumbnail to "{filename}"')
 
     @PostProcessor._restrict_to(images=False)
     def run(self, info):