[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_write_annotations.py.disabled

@@ -1,7 +1,4 @@
 #!/usr/bin/env python3
-# coding: utf-8
-from __future__ import unicode_literals
-
 # Allow direct execution
 import os
 import sys
@@ -21,7 +18,7 @@ import yt_dlp.extractor
 
 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
 
 
@@ -52,7 +49,7 @@ class TestAnnotations(unittest.TestCase):
         ydl.download([TEST_ID])
         self.assertTrue(os.path.exists(ANNOTATIONS_FILE))
         annoxml = None
-        with io.open(ANNOTATIONS_FILE, 'r', encoding='utf-8') as annof:
+        with open(ANNOTATIONS_FILE, encoding='utf-8') as annof:
             annoxml = xml.etree.ElementTree.parse(annof)
         self.assertTrue(annoxml is not None, 'Failed to parse annotations XML')
         root = annoxml.getroot()