Migrate to Python 3 > 2019-01-01

[rsvp]

[... continued from fecon235, https://github.com/rsvp/fecon235/issues/9]

Home for our Jupyter notebooks shall remain at fecon235 due to their bulky size. Migration to the python3 kernel should present no problems since fecon235 has been compatible with both python27 and python3.

Version 10 of fecon236 represents the refactoring of only the fecon235 v5.18.0312 library code (not the Jupyter notebooks). It shall maintain compatibility with both python27 and python34.

After 2019-01-01, our official support for python27 will discontinue (following policy at numpy and pandas), however, straddling code may still continue to work.

Version 11 of fecon236 will signal when our Travis builds under python27 fail, and at that point we expect to require at least Python 3.6.