Add typing to from_parent return values

[bluetech]

Up to now the return values of from_parent were untyped, this is an attempt to make it work with typing.Self.

[Zac-HD]

Looks great - thanks @bluetech!

[Avasam]

@bluetech This breaks the return type on Python 3.9 and under. You should be using from typing_extension import Self instead.

For example, the following code:

def pytest_collect_file(file_path: pathlib.Path, parent: Node) -> CheckdocsItem | None:
    if file_path.name not in project_files:
        return None
    return CheckdocsItem.from_parent(parent, name='project')

Will error with Returning Any from function declared to return "CheckdocsItem | None" [no-any-return] on 3.8 & 3.9, but pass on 3.10+ Which can only be worked around by disabling the error entirely in mypy, or doing:

def pytest_collect_file(file_path: pathlib.Path, parent: Node) -> CheckdocsItem | None:
    if file_path.name not in project_files:
        return None
    if sys.version_info >= (3, 10):  # pytest-dev/pytest#
        return CheckdocsItem.from_parent(parent, name='project')
    else:
        return cast(CheckdocsItem, CheckdocsItem.from_parent(parent, name='project'))