[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

test/test_post_hooks.py

@@ -1,7 +1,4 @@
 #!/usr/bin/env python3
-
-from __future__ import unicode_literals
-
 import os
 import sys
 import unittest
@@ -14,7 +11,7 @@ from yt_dlp.utils import DownloadError
 
 class YoutubeDL(yt_dlp.YoutubeDL):
     def __init__(self, *args, **kwargs):
-        super(YoutubeDL, self).__init__(*args, **kwargs)
+        super().__init__(*args, **kwargs)
         self.to_stderr = self.to_screen