Log Message:
py-djangocms-picture: updated to 2.4.0

2.4.0:
Added support for Django 3.0
Pinned django-filer to 1.5.0
Added further tests to raise coverage
Fixed smaller issues found during testing
Dropped support for django-filer <= 1.4
Fixed alt attribute not rendering correctly

2.3.0:
Fixes an issue where get_link doesn't return external picture
Fixes img_srcset_data being processed on an external picture
Added tests for the plugin itself
Updated translations

2.2.1:
Fixed a regression where external images are not shown anymore

2.2.0:
Added support for Django 2.2 and django CMS 3.7
Removed support for Django 2.0
Fixed an issue when the image reference is lost
Extended test matrix
Added isort and adapted imports
Adapted code base to align with other supported addons