Fixed compatibility with ply 3.6

In newly released ply 3.6 function yacc() of the yacc module no longer
accepts 'None' as the 'tabmodule' keyword parameter and expects a valid
string instead.
Meanwhile, the tabmodule file is not created unless write_tables is set
to True.

This patchset provides such a string and ensures that it is unique.
Omitting the 'tabmodule' parameter at all could possibly do the trick as
well, however that would mean that the parameter would get its default
value - 'parsetab' - which could potentially lead to a minor security
vulnerability if the the system somehow had got a malicious 'parsetab'
module (as yacc attempts to load the generated tables from module
specified by 'tabmodule' before proceeding to grammar generation).
Assigning a random value to this parameter prevents such a situation.

This patch also fixes the ply requirement to <=3.6 to prevent future
gate breakages due to unexpected changes in ply.

Closes-Bug: #1448990

Change-Id: I4364729223521864f7cda5a932503d7ce1b6fa36

requirements.txt

@@ -1,5 +1,5 @@
 pbr>=0.6,!=0.7,<1.0
 Babel>=0.9.6
 
-ply
-six>=1.7.0
+ply<=3.6
+six>=1.7.0

yaql/language/factory.py

@@ -14,6 +14,7 @@
 
 import collections
 import re
+import uuid
 
 from ply import lex
 from ply import yacc
@@ -227,6 +228,6 @@ def create(self, options=None):
         ply_parser = yacc.yacc(
             module=self._create_parser(lexer_rules, operators),
             debug=False if not options else options.get("yaql.debug", False),
-            tabmodule=None, write_tables=False)
+            tabmodule='m' + uuid.uuid4().hex, write_tables=False)
 
         return YaqlEngine(ply_lexer, ply_parser, options, self)