Update development status as Production/Stable

This has been used by widely-used projects for over two years
and the most recent release included mostly maintenance PRs.

Author: jarrodmillman

pyproject.toml

@@ -10,7 +10,7 @@ readme = "README.md"
 license = {file = "LICENSE.md"}
 dynamic = ['version']
 classifiers = [
-  "Development Status :: 4 - Beta",
+  "Development Status :: 5 - Production/Stable",
   "License :: OSI Approved :: BSD License",
   "Programming Language :: Python :: 3",
   "Programming Language :: Python :: 3.7",