[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/movefilesafterdownload.py

@@ -1,4 +1,3 @@
-from __future__ import unicode_literals
 import os
 import shutil
 
@@ -47,7 +46,7 @@ class MoveFilesAfterDownloadPP(PostProcessor):
                         % (oldfile, newfile))
                     continue
             make_dir(newfile, PostProcessingError)
-            self.to_screen('Moving file "%s" to "%s"' % (oldfile, newfile))
+            self.to_screen(f'Moving file "{oldfile}" to "{newfile}"')
             shutil.move(oldfile, newfile)  # os.rename cannot move between volumes
 
         info['filepath'] = finalpath