(core) Simple Python 3 compatibility changes

Summary: Changes that move towards python 3 compatibility that are easy to review without much thought

Test Plan: The tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2873

sandbox/grist/textbuilder.py

@@ -11,6 +11,8 @@ import bisect
 import re
 from collections import namedtuple
 
+import six
+
 Patch = namedtuple('Patch', ('start', 'end', 'old_text', 'new_text'))
 
 line_start_re = re.compile(r'^', re.M)
@@ -176,4 +178,4 @@ class Combiner(Builder):
     offset = self._offsets[start_index - 1]
     part = self._parts[start_index - 1]
     in_patch = Patch(patch.start - offset, patch.end - offset, patch.old_text, patch.new_text)
-    return None if isinstance(part, basestring) else part.map_back_patch(in_patch)
+    return None if isinstance(part, six.string_types) else part.map_back_patch(in_patch)