Update nunjucks requirement from ^2.3.0 to ^3.1.6 (#38)
Update nunjucks requirement from ^2.3.0 to ^3.1.6 (#38)

Updates the requirements on [nunjucks](https://github.com/mozilla/nunjucks) to permit the latest version.
- [Release notes](https://github.com/mozilla/nunjucks/releases)
- [Changelog](https://github.com/mozilla/nunjucks/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mozilla/nunjucks/commits/v3.1.6)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
hexojs/hexo-generator-sitemap