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[unittest/python] add test_struct

+ do not test slice for std::array<Eigen..., k>

Add example showing how to bind virtual classes, passing to overrides by reference, registering shared_ptr, and overriding factories and data classes from Python

+ unittest: make bind_optional and test_optional.py into template files
+ cmake: instantiate boost and std optional tests using configure_file

+ default is boost::optional<T>
+ add test for boost::optional

+ create struct with fixed-size members, expose with setter

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- [github.com/psf/black: 22.12.0 → 23.1.0](https://github.com/psf/black/compare/22.12.0...23.1.0)

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

& update its docstring

* fix includes
* comment out edge cases

unittest/eigen_ref.cpp: asConstRef should take input as Ref<const T>

eigen-allocator: try a fix for mutable Eigen::Ref from Python array with "wrong" layout

* trick transposes pyArray to get the right layout :)