Add anyset & frozenset, enable copying (cast) to std::set (#3901)

* 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>

tests/test_stl.py

@@ -73,6 +73,7 @@ def test_set(doc):
     assert s == {"key1", "key2"}
     s.add("key3")
     assert m.load_set(s)
+    assert m.load_set(frozenset(s))
 
     assert doc(m.cast_set) == "cast_set() -> Set[str]"
     assert doc(m.load_set) == "load_set(arg0: Set[str]) -> bool"