build(deps): Bump pytz from 2019.2 to 2019.3 in /bot

Bumps [pytz](https://github.com/stub42/pytz) from 2019.2 to 2019.3.
- [Release notes](https://github.com/stub42/pytz/releases)
- [Commits](stub42/pytz@release_2019.2...release_2019.3)

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

Author: dependabot-preview[bot]

bot/requirements.txt

@@ -2,5 +2,5 @@
 google-cloud-storage==1.20.0
 libmozdata==0.1.59
 pytoml==0.1.21
-pytz==2019.2
+pytz==2019.3
 zstandard==0.12.0