Export types of builtin fixture for type annotations

In order to allow users to type annotate fixtures they request, the
types need to be imported from the `pytest` namespace. They are/were
always available to import from the `_pytest` namespace, but that is
not guaranteed to be stable.

These types are only exported for the purpose of typing. Specifically,
the following are *not* public:

- Construction (`__init__`)
- Subclassing
- staticmethods and classmethods

We try to combat them being used anyway by:

- Marking the classes as `@final` when possible (already done).

- Not documenting private stuff in the API Reference.

- Using `_`-prefixed names or marking as `:meta private:` for private
  stuff.

- Adding a keyword-only `_ispytest=False` to private constructors,
  warning if False, and changing pytest itself to pass True. In the
  future it will (hopefully) become a hard error.

Hopefully that will be enough.

src/_pytest/python.py

@@ -1620,7 +1620,7 @@ class Function(PyobjMixin, nodes.Item):
 
     def _initrequest(self) -> None:
         self.funcargs: Dict[str, object] = {}
-        self._request = fixtures.FixtureRequest(self)
+        self._request = fixtures.FixtureRequest(self, _ispytest=True)
 
     @property
     def function(self):