pyproject(tool.mypy[python_version]) Add 3.9

See also: https://mypy.readthedocs.io/en/stable/config_file.html#confval-python_version

Author: tony

pyproject.toml

@@ -126,6 +126,7 @@ build-backend = "hatchling.build"
 
 [tool.mypy]
 strict = true
+python_version = "3.9"
 files = [
   "src/",
   "tests/",