Add force_str to FeaturedPostsPlugin __str__ method

[protoroto]

Description

Add force_str to FeaturedPostsPlugin str method

References

Fix #780

Checklist

• [x] I have read the contribution guide
• [x] Code lint checked via inv lint
• [x] changes file included (see docs)
• [x] Usage documentation added in case of new features
• [ ] Tests added

[coveralls]

coverage: 97.167%. remained the same when pulling 4f5df32e07747ac1c0d6a17d36da78af578014cf on protoroto:bugfix/issue-780-fix-missing-force-str into d95802c1475fc7e06ba121a178bb0e755be9bba9 on nephila:develop.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d95802c) 94.38% compared to head (4f5df32) 94.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #782 +/- ## ======================================== Coverage 94.38% 94.38% ======================================== Files 26 26 Lines 1871 1871 Branches 266 266 ======================================== Hits 1766 1766 Misses 53 53 Partials 52 52 ``` | [Flag](https://app.codecov.io/gh/nephila/djangocms-blog/pull/782/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/nephila/djangocms-blog/pull/782/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila) | `94.38% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila#carryforward-flags-in-the-pull-request-comment) to find out more.

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