Bump pytz from 2020.4 to 2020.5

Bumps [pytz](https://github.com/stub42/pytz) from 2020.4 to 2020.5.
- [Release notes](https://github.com/stub42/pytz/releases)
- [Commits](stub42/pytz@release_2020.4...release_2020.5)

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

requirements/base.txt

@@ -12,7 +12,7 @@ gunicorn==20.0.4
 gviz_api==1.9.0
 py-bcrypt==0.4
 psycopg2-binary==2.8.6
-pytz==2020.4
+pytz==2020.5
 static==1.1.1
 whitenoise[brotli]==5.2.0
 future==0.18.2