Update pytz from 2017.3 to 2018.3

Author: pyup-bot

requirements.txt

@@ -41,7 +41,7 @@ Pygments==2.2.0
 pytest==3.3.2
 pytest-runner==3.0
 python-dateutil==2.6.1
-pytz==2017.3
+pytz==2018.3
 PyYAML==3.12
 requests==2.18.4
 setuptools_scm>=1.15.0