(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/urllib_patch.py

@@ -0,0 +1,16 @@
+import urllib
+
+import six
+from six.moves import urllib_parse
+
+original_quote = urllib_parse.quote
+
+def patched_quote(s, safe='/'):
+  if isinstance(s, six.text_type):
+    s = s.encode('utf8')
+  result = original_quote(s, safe=safe)
+  if isinstance(result, six.binary_type):
+    result = result.decode('utf8')
+  return result
+
+urllib.quote = patched_quote