Bump to v5.9.0 (#112)

Author: cosimon

setup.py

@@ -4,7 +4,7 @@
 import subprocess
 
 setup(name="singer-python",
-      version='5.8.1',
+      version='5.9.0',
       description="Singer.io utility library",
       author="Stitch",
       classifiers=['Programming Language :: Python :: 3 :: Only'],