Stop saying mypy is beta software (#14251)

Ref #13685 

cc @Michael0x2a @JukkaL @hauntsaninja

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

README.md

@@ -160,14 +160,7 @@ To get started with developing mypy, see [CONTRIBUTING.md](CONTRIBUTING.md).
 If you need help getting started, don't hesitate to ask on [gitter](https://gitter.im/python/typing).
 
 
-Development status
-------------------
-
-Mypy is beta software, but it has already been used in production
-for several years at Dropbox and in many other organizations, and
-it has an extensive test suite.
-
-mypyc and compiled version of mypy
+Mypyc and compiled version of mypy
 ----------------------------------
 
 [Mypyc](https://github.com/mypyc/mypyc) uses Python type hints to compile Python

docs/source/index.rst

@@ -39,10 +39,10 @@ understand, debug, and maintain.
 
 .. note::
 
-   Mypy is used in production by many companies and projects, but mypy is
-   officially beta software. There will be occasional changes
+   Although mypy is production ready, there will be occasional changes
    that break backward compatibility. The mypy development team tries to
-   minimize the impact of changes to user code.
+   minimize the impact of changes to user code. In case of a major breaking
+   change, mypy's major version will be bumped.
 
 Contents
 --------

setup.py

@@ -171,7 +171,7 @@ def run(self):
 
 
 classifiers = [
-    "Development Status :: 4 - Beta",
+    "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
     "Intended Audience :: Developers",
     "License :: OSI Approved :: MIT License",