sqlalchemy: fix TypeMixer.get_default for Sequences

if a column had a Sequence defined, TypeMixer.get_default thrown an exception because 'arg' attribute doesn't exist on column.default. Return NO_VALUE in such cases.
minor refactoring
Read version carefully
Move magic objects to mixer types.
Remove six.
Some tests refactoring
Move factories.
Relation process refactoring.
Merge branch 'feature/refatoring' into develop
Middleware support
Update version
Merge branch 'release/3.0.0' into develop