setup.py: Mark diffoscope as stable. It is, at least, not "Alpha" anymore.

setup.py

@@ -62,7 +62,7 @@ setup(
     extras_require=extras_require,
     python_requires=">=3.7",
     classifiers=[
-        "Development Status :: 3 - Alpha",
+        "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
         "Operating System :: POSIX",