build(deps): bump zstandard from 0.14.1 to 0.15.1 in /bot

Bumps [zstandard](https://github.com/indygreg/python-zstandard) from 0.14.1 to 0.15.1.
- [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@0.14.1...0.15.1)

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

Author: dependabot[bot]

bot/requirements.txt

@@ -6,4 +6,4 @@ pytz==2020.4
 pyyaml==5.3.1
 tenacity==6.3.1
 tqdm==4.56.0
-zstandard==0.14.1
+zstandard==0.15.1