Use Node.from_parent to instantiate MypyItems

[dmtucker]

Resolve #81

I ran into pytest-dev/pytest#6928 and found this advice particularly helpful:

the structurally correct solution is to have a File that collects a Item

I implemented it by adapting the example for specifying tests in Yaml files.

Notable effects include:

• MypyFileItem is no longer a pytest.File.
• nodeid_name can no longer be None.