Let configure the files analyzed by mypy from configuration file

Describe the bug

Mypy reads the files to analyse in the configuration file, when not set on the command line. https://mypy.readthedocs.io/en/stable/config_file.html#confval-files

Using the configuration file allow a single configuration shared between local development environment and CI/CD environment.

Expected behavior

By default, if the mypy configuration file is correctly configured, the files to analyse are picked from this config file.

Actual behavior

Currently, if the MYPY_FILES is not set or empty, a $(find...) command is triggered.