Add mypy_version to config_type

Feature

Propose to add mypy_version similar to python_version in config_type from mypy/config_parser.py - to indicate which mypy version is recommended when parsing a config file.

Pitch

When developer create the mypy config file, there's no way to indicate which mypy version was used during the development. This could create confusion when different developers installed different versions of mypy while working on the same project.

with mypy_version one can specify the agreed upon mypy_version to be used when parsing a config file. It should print warning/error message when the desired mypy version specified is not the same as the one installed.

Alternative

One could use the package manager config to specify desire mypy version, e.g. requirements.txt. but it doesn't warn developer when a wrong version was installed/used.

Start in mypy/config_parser.py at config_type and compare the proposed setting with python_version handling. Trace how configuration values are parsed and how version diagnostics are reported, then clarify whether a mismatch should produce a warning or an error. Done means matching and mismatching mypy versions are handled and covered by parser tests.