Add mypy configuration file

.mypy.ini:
As mypy can not yet be configured through pyproject.toml, add a specific
configuration file for it.

.mypy.ini

+[mypy]
+ignore_missing_imports = True
+follow_imports = silent
+follow_imports_for_stubs = True
+warn_unused_ignores = True
+warn_no_return = True
+warn_return_any = True
+warn_incomplete_stub = True
+warn_redundant_casts = True
+warn_unused_configs = True
+no_implicit_optional = True
+warn_unreachable = True
+check_untyped_defs = True
+disallow_any_generics = True
+disallow_untyped_calls = True
+disallow_untyped_defs = True
+disallow_incomplete_defs = True