Bump flask-sitemap from 0.3.0 to 0.4.0 (#158)

Bumps [flask-sitemap](https://github.com/inveniosoftware/flask-sitemap) from 0.3.0 to 0.4.0.
- [Release notes](https://github.com/inveniosoftware/flask-sitemap/releases)
- [Changelog](https://github.com/inveniosoftware/flask-sitemap/blob/master/CHANGES)
- [Commits](inveniosoftware/flask-sitemap@v0.3.0...v0.4.0)

---
updated-dependencies:
- dependency-name: flask-sitemap
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -8,7 +8,7 @@ Flask-Limiter==1.4
 Flask-Login==0.6.1
 Flask-Mail==0.9.1
 Flask-Migrate==3.0.1
-Flask-Sitemap==0.3.0
+Flask-Sitemap==0.4.0
 Flask-SQLAlchemy==2.5.1
 Flask-WTF==0.15.1
 future==0.18.2