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/get_platforms.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python
-from __future__ import print_function
 
 import os
 import sys