Update mypy.ini

Disable implicit optional checks in mypy.ini

- [PR to PEP that changed the behavior](python/peps#689)
- [mypy option](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-optional)

{{cookiecutter.project_slug}}/mypy.ini

@@ -2,6 +2,8 @@
 python_version = 3.7
 follow_imports = silent
 scripts_are_modules = true
+namespace_packages = true
+no_implicit_optional = true
 
 # We had to ignore missing imports, because of third-party libraries installed
 # inside the virtualenv, and apparently there's no easy way for mypy to respect