Unify PYSON string format between all clients

issue9723
review328481002

CHANGELOG

@@ -1,3 +1,5 @@
+* Unify PYSON string format
+
 Version 5.8.0 - 2020-11-02
 * Bug fixes (see mercurial logs for details)
 * Remove support for Python 3.5

proteus/pyson.py

@@ -302,8 +302,8 @@ class Greater(PYSON):
         super(Greater, self).__init__()
         for i in (statement1, statement2):
             if isinstance(i, PYSON):
-                assert i.types().issubset(
-                    {int, float, type(None),
+                assert i.types().issubset({
+                        int, float, type(None),
                         datetime.datetime, datetime.date}), \
                     'statement must be an integer, float, date or datetime'
             else:
@@ -747,4 +747,6 @@ CONTEXT = {
     'DateTime': DateTime,
     'TimeDelta': TimeDelta,
     'Len': Len,
+    'true': True,
+    'false': False,
 }