[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/extractor/anvato_token_generator/common.py

@@ -1,6 +1,3 @@
-from __future__ import unicode_literals
-
-
 class TokenGenerator:
     def generate(self, anvack, mcp_id):
         raise NotImplementedError('This method must be implemented by subclasses')