Duplicate "alt" attribute on images in default template

[pakal]

It's a detail, but since the default template (picture.html) uses "{{ instance.attributes_str }}" after already outputting a potential "{{ instance.attributes.alt }}", it gives a duplicated "alt" attribute on final html tag.

I guess it could be solved just by tweaking the "excluded_keys" parameter of the AbstractPicture model's "attributes" field?

Tested on djangocms-picture==2.4.0, but today's master has identical code on these parts so I guess this tiny bug is still there.