Bump pytz from 2017.2 to 2019.1

Bumps [pytz](https://github.com/stub42/pytz) from 2017.2 to 2019.1.
- [Release notes](https://github.com/stub42/pytz/releases)
- [Commits](stub42/pytz@release_2017.2...release_2019.1)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -23,7 +23,7 @@ prettyconf==1.2.3
 psycopg2==2.7.1
 pur==4.0.2
 pytest==3.0.7
-pytz==2017.2
+pytz==2019.1
 raven[flask]==6.0.0
 SQLAlchemy==1.1.6