New features in 2.6.x:
Keyword-only arguments supported in Python 2 or 3 with py::kw_only().
Positional-only arguments supported in Python 2 or 3 with py::pos_only().
py::is_final() class modifier to block subclassing (CPython only).
Added py::prepend(), allowing a function to be placed at the beginning of the overload chain.
Access to the type object now provided with py::type::of<T>() and py::type::of(h).
Perfect forwarding support for methods.
Added py::error_already_set::discard_as_unraisable().
py::hash is now public.
py::class_<union_type> is now supported.
Classes now check local scope when registering members
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