* Upgrade all packages using pur

for reqs in `ls requirements`;
do
  pur
  --skip django-tastypie,django,docker,elasticsearch,pyelasticsearch,commonmark,stripe,djangorestframework,mkdocs,django-allauth,django-filter,mercurial
  --requirement requirements/$reqs;
done

* Run lint with Python 3.6

* Downgrade gitpython to 2.1.10

* Linting error fixed

* Upgrade common

Branch: rtfd/common#17

* Remove old note for testing

* Add note for django-filter

* no-else-return fixed in all the files

Some block needed to be re-idented to fulfill this check.

Basically, if you have a "if something: return another" the "else"
clause is not needed since it will be the common case for the flow to
continue if the condition is not met.

* Only # noqa comment

* Note about gitpython