Fix minor date formatting issue for date with years < 1000

[matthewhegarty]

Problem

There is a minor bug in DateWidget and DateTimeWidget where dates with years < 1000 are not formatted with 4 digits.
This is a minor issue but is a bug that may affect anyone exporting data with historic dates.

4 digit years are OK

>>> from import_export.widgets import DateWidget
>>> from datetime import date
>>> w = DateWidget()
>>> d = date(1000, 5, 3)
>>> w.render(d)
'1000-05-03'

<4 digit years don't render with leading zeroes

Before fix

>>> d = date(999, 5, 3)
>>> w.render(d)
'999-05-03'

After fix

>>> d = date(999, 5, 3)
>>> w.render(d)
'0999-05-03'

Solution

This PR fixes that by using the datetime_safe module in Django

There was an earlier fix to address dates before 1900, which was a previous issue in Django, however that fix did not address this bug. Also that fix applied only to DateWidget and not DateTimeWidget.

The original Django issue was updated once python2 was unsupported.

Acceptance Criteria

• Added tests which cover this scenario
• Improved widgets.py coverage to 100%

[coveralls]

Coverage increased (+0.5%) to 97.849% when pulling 7826db9 on matthewhegarty:widget-handle-dates-before-1000 into bf4a51d on django-import-export:main.

[andrewgy8]

Thanks @matthewhegarty for the extra test coverage too!