Clean up PY2 stuff and improve code structure

[walterddr]

lots of python2 __future__ import is still in pinot-dbapi.

Several clean ups.

• clean up py2 stuff
• add typing
• split codes in db.py
  • type and column processing into separate file
  • query response processing / data manipulation into separate file

It seems like we still have lots to do to support more sqlalchemy features. but will left that into separate issues, potentially we can leverage the type / column / schema / data utils split out from db.py

[walterddr]

py2 stuff is cleaned up.
ci is added.

next will try to add typing