FixtureRequest.node should be typed to return a Node

Currently the property node of FixtureRequest is defined as

    @property
    @abc.abstractmethod
    def node(self):
         """Underlying collection node (depends on current request scope)."""
         raise NotImplementedError()

which implicitly types the return type as Any. However, based on the name of the function and the underlying implementations it should return a nodes.Node.

Unfortunately, when I tried this on a fork, I got the following error:

mypy............................................................................Failed
- hook id: mypy
- exit code: 1

src/_pytest/junitxml.py:294: error: "Node" has no attribute "user_properties"  [attr-defined]
Found 1 error in 1 file (checked 239 source files)

This happens because junitxml actually expects to receive a nodes.NodeItem instead of a plain nodes.Node.

One quick fix for this would be to put a type assertion in junitxml, but I'm not sure if that's the preferred approach. I'd be happy to send a PR if someone could give ve some guidance on how to best resolve this.

Start by inspecting the FixtureRequest.node declaration and its underlying implementations, then read src/_pytest/junitxml.py around line 294. Run the mypy check and determine how the Node versus NodeItem expectation should be represented; done means the return type is accurate and mypy passes without the reported user_properties error.