Allow use of xlsxwriter 2.0 and 3.0.

Update xlsxwriter constraint to allow usage of xlsxwriter 2.0 and 3.0.
The latter version includes significant improvements to xlsxwriter's
performance.

setup.py

@@ -33,7 +33,7 @@ install_requires = [
     "Willow>=1.4,<1.5",
     "requests>=2.11.1,<3.0",
     "l18n>=2018.5",
-    "xlsxwriter>=1.2.8,<2.0",
+    "xlsxwriter>=1.2.8,<4.0",
     "tablib[xls,xlsx]>=0.14.0",
     "anyascii>=0.1.5",
     "telepath>=0.1.1,<1",