workaround for application bundling tools (#3235)

* cx_freeze support for default_backend

* updated tabing to spaces

* corrected spacing

* moved finding backend to backends __init__

* update to check to see if sys is frozen

* corrected pep8 issues

* update based on comments

* changes to simplify, support testing, and improve comments

* add changelog entry

* right, coverage. I remember now. Time for some contortions.

* updated with review feedback
3 files changed