Log Message:
py-parsedatetime: updated to 2.6

v2.6 release
Polished README.rst
fix pyicu import to suppress warnings
Fixed missing comma in seconds strings

Updated Pipfile and Makefile to:
- update and move packages to the "dev" section
- use Python 3.7 for pipenv
- install tox-pipenv plugin to try and fix Tox (currently doesn't)
- simplify tox.ini to try and fix Tox (didn't)
- move ci makefile target to the circle config