[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/cache.py

@@ -1,7 +1,4 @@
-from __future__ import unicode_literals
-
 import errno
-import io
 import json
 import os
 import re
@@ -15,7 +12,7 @@ from .utils import (
 )
 
 
-class Cache(object):
+class Cache:
     def __init__(self, ydl):
         self._ydl = ydl
 
@@ -31,7 +28,7 @@ class Cache(object):
             'invalid section %r' % section
         assert re.match(r'^[a-zA-Z0-9_.-]+$', key), 'invalid key %r' % key
         return os.path.join(
-            self._get_root_dir(), section, '%s.%s' % (key, dtype))
+            self._get_root_dir(), section, f'{key}.{dtype}')
 
     @property
     def enabled(self):
@@ -54,8 +51,7 @@ class Cache(object):
             write_json_file(data, fn)
         except Exception:
             tb = traceback.format_exc()
-            self._ydl.report_warning(
-                'Writing cache to %r failed: %s' % (fn, tb))
+            self._ydl.report_warning(f'Writing cache to {fn!r} failed: {tb}')
 
     def load(self, section, key, dtype='json', default=None):
         assert dtype in ('json',)
@@ -66,17 +62,16 @@ class Cache(object):
         cache_fn = self._get_cache_fn(section, key, dtype)
         try:
             try:
-                with io.open(cache_fn, 'r', encoding='utf-8') as cachef:
+                with open(cache_fn, encoding='utf-8') as cachef:
                     self._ydl.write_debug(f'Loading {section}.{key} from cache')
                     return json.load(cachef)
             except ValueError:
                 try:
                     file_size = os.path.getsize(cache_fn)
-                except (OSError, IOError) as oe:
+                except OSError as oe:
                     file_size = str(oe)
-                self._ydl.report_warning(
-                    'Cache retrieval from %s failed (%s)' % (cache_fn, file_size))
-        except IOError:
+                self._ydl.report_warning(f'Cache retrieval from {cache_fn} failed ({file_size})')
+        except OSError:
             pass  # No cache available
 
         return default