Fix non-dynamic attributes on cvars
* Fixes parsing of non-dynamic attributes in cvars by wrabit in https://github.com/wrabit/django-cotton/pull/251
Previously we were still template-parsing non-dynamic attributes on `<c-vars />`. So this:
`<c-vars attribute1="None" attribute2="False" attribute3="1" />`
Would give us the python types `None`, `False`, `1` respectively. Even though if these were attributes on the component, they would have correctly been provided as strings. As raised in 249.
**Full Changelog**: https://github.com/wrabit/django-cotton/compare/v1.5.2...1.5.3