Updated dj.py for supporting latest Django

[nhridoy]

Change One:
restless/restless/dj.py
from django.conf.urls import url ❌
from django.urls import re_path ✔️

url(r'^$', cls.as_list(), name=cls.build_url_name('list', name_prefix)), ❌
re_path (r'^$', cls.as_list(), name=cls.build_url_name('list', name_prefix)), ✔️

url(r'^(?P<pk>[\w-]+)/$', cls.as_detail(), name=cls.build_url_name('detail', name_prefix)), ❌
re_path (r'^(?P<pk>[\w-]+)/$', cls.as_detail(), name=cls.build_url_name('detail', name_prefix)), ✔️

Change Two:
Refactor codes

[tianfan]

I am wondering if this pull request done? Can the admin merge the pull request? this is an important change.

[jeromelefeuvre]

More than 1 years and not yet merged! Hey @toastdriven, can you do something or add more collaborators to merge this! Thanks!!