(core) Remaining Python 3 compatibility changes

Summary: Biggest change is turning everything to unicode

Test Plan: The tests

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2875

sandbox/grist/autocomplete_context.py

@@ -70,6 +70,9 @@ class AutocompleteContext(object):
     return self._context
 
   def process_result(self, result):
+    # 'for' suggests the autocompletion 'for ' in python 3
+    result = result.rstrip()
+
     # Callables are returned by rlcompleter with a trailing "(".
     if result.endswith('('):
       funcname = result[0:-1]