nodes: deprecate fspath arguments to node constructors

This is unfortunately a dependency on `py.path` which cannot be moved to
an external plugins or eased in any way, so has to be deprecated in
order for pytest to be able to eventually remove the dependency on `py`.

testing/deprecated_test.py

@@ -215,3 +215,16 @@ def test_deprecation_of_cmdline_preparse(pytester: Pytester) -> None:
             "*Please use pytest_load_initial_conftests hook instead.*",
         ]
     )
+
+
+def test_node_ctor_fspath_argument_is_deprecated(pytester: Pytester) -> None:
+    mod = pytester.getmodulecol("")
+
+    with pytest.warns(
+        pytest.PytestDeprecationWarning,
+        match=re.escape("The (fspath: py.path.local) argument to File is deprecated."),
+    ):
+        pytest.File.from_parent(
+            parent=mod.parent,
+            fspath=legacy_path("bla"),
+        )