Clean up Django 2 path backslashes

In Django 2, routes defines via urls.path are aggresively escaped when converted into regex.

This is a naive fix which unescapes all characters outside capture groups, but in the context of OpenAPI is okay because regular expressions inside paths are not supported anyway.

This issue affects django-rest-framework as well, as outlined in encode/django-rest-framework#5672, encode/django-rest-framework#5675.

testproj/users/urls.py

@@ -4,5 +4,5 @@ from users import views
 
 urlpatterns = [
     url(r'^$', views.UserList.as_view()),
-    url(r'^(?P<pk>[0-9]+)/$', views.user_detail),
+    url(r'^(?P<pk>\d+)/$', views.user_detail),
 ]