Document project as stable and ready for use in production (#4284)

diff --git a/setup.py b/setup.py
index 1332589..593925e 100644
--- a/setup.py
+++ b/setup.py
@@ -256,6 +256,7 @@
     author_email=about["__email__"],
 
     classifiers=[
+        "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: Apache Software License",
         "License :: OSI Approved :: BSD License",