Updating development status past 3

Per the description here https://martin-thoma.com/software-development-stages/ it seems like we're in at least "5 - Production/Stable" and maybe "6 - Mature". I'm putting this up as a PR instead of committing directly so that anyone interested can discuss before we update. 

Addresses @rfaircloth-splunk 's #385

Author: tdhellmann

setup.py

@@ -234,7 +234,7 @@ def run(self):
 
     classifiers = [
         "Programming Language :: Python",
-        "Development Status :: 3 - Alpha",
+        "Development Status :: 6 - Mature",
         "Environment :: Other Environment",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: Apache Software License",