Drop support for retired Python 3.5

Upgrade syntax using `pyupgrade --py36-plus` .

Manually drop several `six` imports.

Also:
- Remove broken pr_cache in scripts/bootstrap_history.py
- Fix broken prefix removal in lib/galaxy/tool_util/deps/mulled/mulled_build.py

scripts/api/common.py

@@ -1,14 +1,15 @@
 """
 Common methods used by the API sample scripts.
 """
-from __future__ import print_function
 
 import json
 import logging
 import sys
-
-from six.moves.urllib.error import HTTPError
-from six.moves.urllib.request import Request, urlopen
+from urllib.error import HTTPError
+from urllib.request import (
+    Request,
+    urlopen,
+)
 
 log = logging.getLogger(__name__)
 
@@ -92,7 +93,7 @@ def display(api_key, url, return_formatted=True):
             if 'name' in i:
                 print('  name: %s' % i.pop('name'))
             for k, v in i.items():
-                print('  %s: %s' % (k, v))
+                print(f'  {k}: {v}')
         print('')
         print('%d element(s) in collection' % len(r))
     elif type(r) == dict:
@@ -100,7 +101,7 @@ def display(api_key, url, return_formatted=True):
         print('Member Information')
         print('------------------')
         for k, v in r.items():
-            print('%s: %s' % (k, v))
+            print(f'{k}: {v}')
     elif type(r) == str:
         print(r)
     else:
@@ -137,7 +138,7 @@ def submit(api_key, url, data, return_formatted=True):
                 if 'name' in i:
                     print('  name: %s' % i.pop('name'))
                 for k, v in i.items():
-                    print('  %s: %s' % (k, v))
+                    print(f'  {k}: {v}')
             else:
                 print(i)
     else: