[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_compat.py

@@ -1,8 +1,4 @@
 #!/usr/bin/env python3
-# coding: utf-8
-
-from __future__ import unicode_literals
-
 # Allow direct execution
 import os
 import sys
@@ -48,7 +44,7 @@ class TestCompat(unittest.TestCase):
         all_names = yt_dlp.compat.__all__
         present_names = set(filter(
             lambda c: '_' in c and not c.startswith('_'),
-            dir(yt_dlp.compat))) - set(['unicode_literals'])
+            dir(yt_dlp.compat))) - {'unicode_literals'}
         self.assertEqual(all_names, sorted(present_names))
 
     def test_compat_urllib_parse_unquote(self):