build(deps): Bump zstandard from 0.12.0 to 0.13.0 in /bot (#347)

Bumps [zstandard](https://github.com/indygreg/python-zstandard) from 0.12.0 to 0.13.0.
- [Release notes](https://github.com/indygreg/python-zstandard/releases)
- [Changelog](https://github.com/indygreg/python-zstandard/blob/master/NEWS.rst)
- [Commits](indygreg/python-zstandard@0.12.0...0.13.0)

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

Author: dependabot-preview[bot]

bot/requirements.txt

@@ -3,4 +3,4 @@ google-cloud-storage==1.23.0
 libmozdata==0.1.63
 pytoml==0.1.21
 pytz==2019.3
-zstandard==0.12.0
+zstandard==0.13.0