Rename "Python Lint" to "Mypy" and "python_lint.yml" to "mypy.yml"

[Jingru923]

What
We have a pipeline has a name that is not clear enough to show its functionality.

Why
The pipeline is named as Python Lint and the yaml file is named "python_lint.yml". Python lint in general check the error in the code and modify the code automatically based on the code standard. However, we only use one function called Mypy in the pipeline because this is not able to be done during the pre-commit check while other linting activities are done during the pre-commit check. As the name of the pipeline need to show the functionality in a direct way, having the pipeline named Lint is confusing.

How
As the title suggests, change the name "Python Lint" to "Mypy" and "python_lint.yml" to "mypy.yml".