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/auth/pam_auth_helper.py

@@ -16,7 +16,7 @@ except ImportError:
 
 
 def handle_timeout(signum, stack):
-    raise IOError("Timed out reading input")
+    raise OSError("Timed out reading input")
 
 
 # set timeout so we don't block on reading stdin
@@ -32,10 +32,10 @@ signal.alarm(0)
 p_auth = pam.pam()
 authenticated = p_auth.authenticate(pam_username, pam_password, service=pam_service)
 if authenticated:
-    log.debug('PAM auth helper: authentication successful for {}'.format(pam_username))
+    log.debug(f'PAM auth helper: authentication successful for {pam_username}')
     sys.stdout.write('True\n')
     sys.exit(0)
 else:
-    log.debug('PAM auth helper: authentication failed for {}'.format(pam_username))
+    log.debug(f'PAM auth helper: authentication failed for {pam_username}')
     sys.stdout.write('False\n')
     sys.exit(1)