Update project status classifier (#292)

The project is now stable and has been used in production for a while,
it reached 1.0.0, so it's time to update the classifier to reflect that.

Author: llucax

pyproject.toml

@@ -16,7 +16,7 @@ readme = "README.md"
 license = { text = "MIT" }
 keywords = ["frequenz", "python", "lib", "library", "channels", "channel"]
 classifiers = [
-  "Development Status :: 3 - Alpha",
+  "Development Status :: 5 - Production/Stable",
   "Intended Audience :: Developers",
   "License :: OSI Approved :: MIT License",
   "Programming Language :: Python :: 3",