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`.

src/_pytest/deprecated.py

@@ -101,6 +101,14 @@ HOOK_LEGACY_PATH_ARG = UnformattedWarning(
     "#py-path-local-arguments-for-hooks-replaced-with-pathlib-path",
 )
 
+NODE_CTOR_FSPATH_ARG = UnformattedWarning(
+    PytestDeprecationWarning,
+    "The (fspath: py.path.local) argument to {node_type_name} is deprecated. "
+    "Please use the (path: pathlib.Path) argument instead.\n"
+    "See https://docs.pytest.org/en/latest/deprecations.html"
+    "#fspath-argument-for-node-constructors-replaced-with-pathlib-path",
+)
+
 WARNS_NONE_ARG = PytestDeprecationWarning(
     "Passing None to catch any warning has been deprecated, pass no arguments instead:\n"
     " Replace pytest.warns(None) by simply pytest.warns()."