Flatdict2

- Fixes for building wheel

- Gracefully fail to install if setuptools is too old

- FIXED deprecation warning from Python 3.9 (40 [nugend](https://github.com/nugend))
- FIXED keep order of received dict and it's nested objects (38 [wsantos](https://github.com/wsantos))
- Drops Python 2 support and Python 3.4

- FIXED sort order with regard to a nested list of dictionaries (33 [wsantos](https://github.com/wsantos))

- FIXED FlatDict.setdefault() to match dict behavior (32 [abmyii](https://github.com/abmyii))
- FIXED empty nested Flatterdict (30 [wsantos](https://github.com/wsantos))
- CHANGED functionality to allow setting and updating nests within iterables (29 [mileslucas](https://github.com/mileslucas))

- FIXED docs generation for readthedocs.io