Use secure HTTPS link for Mypy badge (#14383)

Now that the site is accessible via HTTPS

 - #9919

Fetch the badge using a secure link, especially since this will be used
an example for others. Also, non-secure badges will not be shown in some
contexts, such as DockerHub, so this makes the example both more secure
and more generally useful.

README.md

@@ -8,7 +8,7 @@ Mypy: Static Typing for Python
 [![Build Status](https://github.com/python/mypy/actions/workflows/test.yml/badge.svg)](https://github.com/python/mypy/actions)
 [![Documentation Status](https://readthedocs.org/projects/mypy/badge/?version=latest)](https://mypy.readthedocs.io/en/latest/?badge=latest)
 [![Chat at https://gitter.im/python/typing](https://badges.gitter.im/python/typing.svg)](https://gitter.im/python/typing?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-[![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)
+[![Checked with mypy](https://www.mypy-lang.org/static/mypy_badge.svg)](https://mypy-lang.org/)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![Imports: isort](https://img.shields.io/badge/%20imports-isort-%231674b1?style=flat&labelColor=ef8336)](https://pycqa.github.io/isort/)