New features

New warning DatabaseContainsIncorrectEmptyValue added, it is raised when the required attribute is empty during loading an entity from the database

Bugfixes

Fixes #216: Added Python 3.6 support
Fixes #203: subtranslator should use argnames from parent translator
Change a way aliases in SQL query are generated in order to fix a problem when a subquery alias masks a base query alias
Volatile attribute bug fixed
Fix creation of self-referenced foreign keys - before this Pony didn't create the foreign key for self-referenced attributes
Bug fixed: when required attribute is empty the loading from the database shouldn't raise the validation error. Now Pony raises the warning DatabaseContainsIncorrectEmptyValue
Throw an error with more clear explanation when a list comprehension is used inside a query instead of a generator expression: "Use generator expression (... for ... in ...) instead of list comprehension [... for ... in ...] inside query"