Bump zstandard from 0.19.0 to 0.21.0

Bumps [zstandard](https://github.com/indygreg/python-zstandard) from 0.19.0 to 0.21.0.
- [Release notes](https://github.com/indygreg/python-zstandard/releases)
- [Changelog](https://github.com/indygreg/python-zstandard/blob/main/docs/news.rst)
- [Commits](indygreg/python-zstandard@0.19.0...0.21.0)

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

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

requirements.txt

@@ -8,4 +8,4 @@ taskcluster==49.2.0
 taskcluster-urls==13.0.1
 twine==4.0.2
 yarl==1.9.2
-zstandard==0.19.0
+zstandard==0.21.0