[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

devscripts/make_supportedsites.py

@@ -1,7 +1,4 @@
 #!/usr/bin/env python3
-from __future__ import unicode_literals
-
-import io
 import optparse
 import os
 import sys
@@ -23,11 +20,11 @@ def main():
 
     def gen_ies_md(ies):
         for ie in ies:
-            ie_md = '**{0}**'.format(ie.IE_NAME)
+            ie_md = f'**{ie.IE_NAME}**'
             if ie.IE_DESC is False:
                 continue
             if ie.IE_DESC is not None:
-                ie_md += ': {0}'.format(ie.IE_DESC)
+                ie_md += f': {ie.IE_DESC}'
             search_key = getattr(ie, 'SEARCH_KEY', None)
             if search_key is not None:
                 ie_md += f'; "{ie.SEARCH_KEY}:" prefix'
@@ -40,7 +37,7 @@ def main():
         ' - ' + md + '\n'
         for md in gen_ies_md(ies))
 
-    with io.open(outfile, 'w', encoding='utf-8') as outf:
+    with open(outfile, 'w', encoding='utf-8') as outf:
         outf.write(out)