[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/check-porn.py

@@ -1,6 +1,4 @@
 #!/usr/bin/env python3
-from __future__ import unicode_literals
-
 """
 This script employs a VERY basic heuristic ('porn' in webpage.lower()) to check
 if we are not 'age_limit' tagging some porn site
@@ -29,7 +27,7 @@ for test in gettestcases():
         try:
             webpage = compat_urllib_request.urlopen(test['url'], timeout=10).read()
         except Exception:
-            print('\nFail: {0}'.format(test['name']))
+            print('\nFail: {}'.format(test['name']))
             continue
 
         webpage = webpage.decode('utf8', 'replace')
@@ -39,7 +37,7 @@ for test in gettestcases():
     elif METHOD == 'LIST':
         domain = compat_urllib_parse_urlparse(test['url']).netloc
         if not domain:
-            print('\nFail: {0}'.format(test['name']))
+            print('\nFail: {}'.format(test['name']))
             continue
         domain = '.'.join(domain.split('.')[-2:])
 
@@ -47,11 +45,11 @@ for test in gettestcases():
 
     if RESULT and ('info_dict' not in test or 'age_limit' not in test['info_dict']
                    or test['info_dict']['age_limit'] != 18):
-        print('\nPotential missing age_limit check: {0}'.format(test['name']))
+        print('\nPotential missing age_limit check: {}'.format(test['name']))
 
     elif not RESULT and ('info_dict' in test and 'age_limit' in test['info_dict']
                          and test['info_dict']['age_limit'] == 18):
-        print('\nPotential false negative: {0}'.format(test['name']))
+        print('\nPotential false negative: {}'.format(test['name']))
 
     else:
         sys.stdout.write('.')