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/tool_shed/api/common.py

@@ -1,12 +1,8 @@
-from __future__ import print_function
-
 import json
 import os
 import sys
-
-import six
-from six.moves.urllib.error import HTTPError
-from six.moves.urllib.request import (
+from urllib.error import HTTPError
+from urllib.request import (
     build_opener,
     HTTPRedirectHandler,
     install_opener,
@@ -94,7 +90,7 @@ def display(url, api_key=None, return_formatted=True):
         # Only return the first 1K of errors.
         print(e.read(1024))
         sys.exit(1)
-    if isinstance(r, six.text_type):
+    if isinstance(r, str):
         print('error: %s' % r)
         return None
     if not return_formatted:
@@ -111,7 +107,7 @@ def display(url, api_key=None, 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 isinstance(r, dict):
@@ -119,7 +115,7 @@ def display(url, api_key=None, return_formatted=True):
         print('Member Information')
         print('------------------')
         for k, v in r.items():
-            print('%s: %s' % (k, v))
+            print(f'{k}: {v}')
     else:
         print('response is unknown type: %s' % type(r))
 
@@ -133,7 +129,7 @@ def get(url, api_key=None):
         sys.exit("URL did not return JSON data")
 
 
-def get_api_url(base, parts=[], params=None):
+def get_api_url(base, parts, params=None):
     """Compose and return a URL for the Tool Shed API."""
     if 'api' in parts and parts.index('api') != 0:
         parts.pop(parts.index('api'))
@@ -271,7 +267,7 @@ def submit(url, data, api_key=None, 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: