Use the actual readme as the readme

Fixes #12456

Things to consider:

Our current readme starts with "Got a question?". We could start with a short description instead, copying in our current description paragraph:

Add type annotations to your Python programs, and use mypy to type check them. Mypy is essentially a Python linter on steroids, and it can catch many programming errors by analyzing your program, without actually having to run it. Mypy has a powerful type system with features such as type inference, gradual typing, generics and union types."

Occasionally there is a relative link, like [CONTRIBUTING.md](CONTRIBUTING.md). Do those need to be changed?

Author: wyattscarpenter

pyproject.toml

@@ -19,17 +19,7 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "mypy"
 description = "Optional static typing for Python"
-readme = {text = """
-Mypy -- Optional Static Typing for Python
-=========================================
-
-Add type annotations to your Python programs, and use mypy to type
-check them.  Mypy is essentially a Python linter on steroids, and it
-can catch many programming errors by analyzing your program, without
-actually having to run it.  Mypy has a powerful type system with
-features such as type inference, gradual typing, generics and union
-types.
-""", content-type = "text/x-rst"}
+readme = "README.md"
 authors = [{name = "Jukka Lehtosalo", email = "jukka.lehtosalo@iki.fi"}]
 license = "MIT"
 license-files = ["LICENSE", "mypy/typeshed/LICENSE"]