setup.py: Time to bump the maturity level

kas is being used in production for many years now and maintained for
that purposes. We are late with declaring this officially via the PyPI
classifiers.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

setup.py

@@ -56,11 +56,7 @@
 
     # See https://pypi.python.org/pypi?%3Aaction=list_classifiers
     classifiers=[
-        # How mature is this project? Common values are
-        #   3 - Alpha
-        #   4 - Beta
-        #   5 - Production/Stable
-        'Development Status :: 4 - Beta',
+        'Development Status :: 5 - Production/Stable',
 
         # Indicate who your project is intended for
         'Intended Audience :: Developers',