* Add Python 3.6 and 3.7 and remove old versions 3.3, 3.4 and 3.5.
* Add pypy3.
* Add Django 1.11.x, 2.1.x and 2.2.x and remove old versions 1.8, 1.9
and 1.10.
* Replace pep8 with flake8 and fix errors detected by flake8
accordingly.
Signed-off-by: Chenxiong Qi <qcxhome@gmail.com>