python: export pytest.Metafunc for typing purposes

The type cannot be constructed directly, but is exported for use in type
annotations, since it is reachable through existing public API.

src/pytest/__init__.py

@@ -40,6 +40,7 @@ from _pytest.pytester import Testdir
 from _pytest.python import Class
 from _pytest.python import Function
 from _pytest.python import Instance
+from _pytest.python import Metafunc
 from _pytest.python import Module
 from _pytest.python import Package
 from _pytest.python_api import approx
@@ -95,6 +96,7 @@ __all__ = [
     "Mark",
     "MarkDecorator",
     "MarkGenerator",
+    "Metafunc",
     "Module",
     "MonkeyPatch",
     "Package",