(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/functions/date.py

@@ -1,6 +1,8 @@
 import calendar
 import datetime
 import dateutil.parser
+import six
+
 import moment
 import docmodel
 
@@ -16,7 +18,7 @@ def _make_datetime(value):
     return datetime.datetime.combine(value, datetime.time())
   elif isinstance(value, datetime.time):
     return datetime.datetime.combine(datetime.date.today(), value)
-  elif isinstance(value, basestring):
+  elif isinstance(value, six.string_types):
     return dateutil.parser.parse(value)
   else:
     raise ValueError('Invalid date %r' % (value,))