Add xslxwriter dependency

setup.py

@@ -34,6 +34,7 @@ install_requires = [
     "Willow>=1.3,<1.4",
     "requests>=2.11.1,<3.0",
     "l18n>=2018.5",
+    "xlsxwriter>=1.2.8,<2.0"
 ]
 
 # Testing dependencies