Static type checking with Mypy

If mypy is set to "y", static type checking is added with mypy. If "github_actions is also set to "y", the code is checked with mypy during every workflow that is triggered. The default configuration is as shown below, and can be edited in pyproject.toml.

[tool.mypy]
disallow_untyped_defs = "True"
disallow_any_unimported = "True"
no_implicit_optional = "True"
check_untyped_defs = "True"
warn_return_any = "True"
warn_unused_ignores = "True"
show_error_codes = "True"
exclude = [
    '\.venv',
    '{{cookiecutter.project_name}}',
    'tests'
]