feat(packaging): add support for Python 3.8 (#569)

Release-As: 1.20.0

Closes #568
diff --git a/noxfile.py b/noxfile.py
index bcea1fb..c39f27c 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -64,7 +64,7 @@
     session.run("black", *BLACK_PATHS)
 
 
-@nox.session(python=["2.7", "3.5", "3.6", "3.7"])
+@nox.session(python=["2.7", "3.5", "3.6", "3.7", "3.8"])
 def unit(session):
     session.install(*TEST_DEPENDENCIES)
     session.install(".")