upgrade "development status" from "alpha" to "beta"

Maybe it is even "stable" but after the big rewrite I'd like to wait
for some more time to see if the code really works.

Author: FelixSchwarz

setup.cfg

@@ -13,7 +13,7 @@ license       = MIT
 license_file  = LICENSE.txt
 
 classifiers =
-    Development Status :: 3 - Alpha
+    Development Status :: 4 - Beta
     Intended Audience :: Developers
     License :: OSI Approved :: MIT License
     Programming Language :: Python :: 3