Rename dependency is_resolved to is_pinned

[AyanSinhaMahapatra]

Renaming the dependency attribute is_resolved to is_pinned to more accurately represent this attribute accurately. This is more relevant after the changes in nexB/scancode.io#1066

Reference: https://github.com/nexB/scancode.io/issues/1066

Tasks

• [x] Reviewed contribution guidelines
• [x] PR is descriptively titled 📑 and links the original issue above 🔗
• [x] Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR Run tests locally to check for errors.
• [x] Commits are in uniquely-named feature branch and has no merge conflicts 📁
• [x] Updated documentation pages (if applicable)
• [x] Updated CHANGELOG.rst (if applicable)

[AyanSinhaMahapatra]

Thanks @pombredanne, resolved your comments and merged!