Use Pipenv to manage requirements

pipenv is a tool for managing application dependencies.
(see https://pipenv.readthedocs.io/)

The Pipfile was generated from the existing
requirements.txt (using pipenv install),
and the requirements.txt were generated back
from the Pipfile[.lock]
(using pipenv lock -r > requirements.txt)

Same thing for the requirements-test.txt file
(renamed to requirements-dev for convention)

This will make it easier to bump dependencies.

Change-Id: Iae7ecc0f424dfe488a7f6582608aee29077f7083