Replace 'assert' with 'assert*' method.

Convert tests.unit.forms.widget_tests to use TestCase class.

Convert bankcard tests to use TestCase.

Convert tests.unit.logging_tests to use TestCase.

This also removes the attr() marking of tests. This can be implemented
in pytest with marking the tests with a custom marker if required.

* Document new datetime picker in release notes.
* Update the tests to new format
* Fix some PEP8 issues

This fixes two issues:

1) The code didn't take into account that self.format will be ignored by
   Django if USE_L10N=True, unless a format was specified when constructing
   the Widget. Remedied by moving the code to render because the current
   language is determined at request time, not at object construction time,
   and looking up the correct format there.

2) The code couldn't handle date formats that include dots, like
   31.12.2014.

Fixes #1440.