Drop Python 2.7 support

For a 1.0 release of mappyfile, support for Python 2.7 will be dropped. If there are any good reasons to continue 2.7 support, please add them to this issue. 

Changes:

- allow removal of custom dict classes
- update packaging - see https://packaging.python.org/en/latest/tutorials/packaging-projects

Additional tasks to follow. 
