build(deps): Bump pytz from 2019.1 to 2019.2 in /bot (#93)

Bump [pytz](https://github.com/stub42/pytz) from 2019.1 to 2019.2.
- [Release notes](https://github.com/stub42/pytz/releases)
- [Commits](stub42/pytz@release_2019.1...release_2019.2)

Author: dependabot-preview[bot]

bot/requirements.txt

@@ -2,5 +2,5 @@
 google-cloud-storage==1.16.1
 libmozdata==0.1.57
 pytoml==0.1.21
-pytz==2019.1
+pytz==2019.2
 zstandard==0.11.1