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

tryton/pyson.py

@@ -747,4 +747,6 @@ CONTEXT = {
     'DateTime': DateTime,
     'TimeDelta': TimeDelta,
     'Len': Len,
+    'true': True,
+    'false': False,
 }