feat(typing): allow annotate methods with pos_only when only have the self argument (#5403)

* feat: allow annotate methods with `pos_only` when only have the `self` argument

* chore(typing): make arguments for auto-generated dunder methods positional-only

* docs: add more comments to improve readability

* style: fix nit suggestions

* Add test_self_only_pos_only() in tests/test_methods_and_attributes

* test: add docstring tests for generated dunder methods

* test: remove failed tests

* fix(test): run `gc.collect()` three times for refcount tests

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

tests/test_pickling.py

@@ -93,3 +93,20 @@ def test_roundtrip_simple_cpp_derived():
     # Issue #3062: pickleable base C++ classes can incur object slicing
     #              if derived typeid is not registered with pybind11
     assert not m.check_dynamic_cast_SimpleCppDerived(p2)
+
+
+def test_new_style_pickle_getstate_pos_only():
+    assert (
+        re.match(
+            r"^__getstate__\(self: [\w\.]+, /\)", m.PickleableNew.__getstate__.__doc__
+        )
+        is not None
+    )
+    if hasattr(m, "PickleableWithDictNew"):
+        assert (
+            re.match(
+                r"^__getstate__\(self: [\w\.]+, /\)",
+                m.PickleableWithDictNew.__getstate__.__doc__,
+            )
+            is not None
+        )