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

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