build(deps): bump pillow-heif from 0.20.0 to 0.21.0

Bumps [pillow-heif](https://github.com/bigcat88/pillow_heif) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/bigcat88/pillow_heif/releases)
- [Changelog](https://github.com/bigcat88/pillow_heif/blob/master/CHANGELOG.md)
- [Commits](bigcat88/pillow_heif@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: pillow-heif
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

requirements.txt

@@ -4,5 +4,5 @@ xkcd==2.4.2
 pytz==2024.2
 asyncpg==0.30.0
 python-Levenshtein==0.26.1
-pillow-heif==0.20.0
+pillow-heif==0.21.0
 markdownify==0.14.1