add requirements.txt

Summary:

• it makes easier to install dependencies
• allows to "version control" dependencies

See https://pip.readthedocs.io/en/1.1/requirements.html

Test Plan:

• install virtualenv
• run pip install -r requirements.txt

Reviewers: fabianr, romangg, bshah

Reviewed By: fabianr

Tags: KDE Human Interface Guidelines

Differential Revision: https://phabricator.kde.org/D10677