build(deps): bump pytz from 2020.5 to 2021.1 in /bot

Bumps [pytz](https://github.com/stub42/pytz) from 2020.5 to 2021.1.
- [Release notes](https://github.com/stub42/pytz/releases)
- [Commits](stub42/pytz@release_2020.5...release_2021.1)

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

Author: dependabot[bot]

bot/requirements.txt

@@ -2,7 +2,7 @@
 google-cloud-storage==1.35.0
 libmozdata==0.1.70
 pytoml==0.1.21
-pytz==2020.5
+pytz==2021.1
 pyyaml==5.4.1
 tenacity==6.3.1
 tqdm==4.59.0