* Add frozenset, and allow it cast to std::set
For the reverse direction, std::set still casts to set. This is in concordance with the behavior for sequence containers, where e.g. tuple casts to std::vector but std::vector casts to list.
Extracted from #3886.
* Rename set_base to any_set to match Python C API
since this will be part of pybind11 public API
* PR: static_cast, anyset
* Add tests for frozenset
and rename anyset methods
* Remove frozenset default ctor, add tests
Making frozenset non-default constructible means that we need to adjust pyobject_caster to not require that its value is default constructible, by initializing value to a nil handle. This also allows writing C++ functions taking anyset, and is arguably a performance improvement, since there is no need to allocate an object that will just be replaced by load.
Add some more tests, including anyset::empty, anyset::size, set::add and set::clear.
* [pre-commit.ci] auto fixes from pre-commit.com hooks
for more information, see https://pre-commit.ci
* Add rationale to `pyobject_caster` default ctor
* Remove ineffectual protected: access control
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
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