MSVC C++20 test_eigen

[rwgk]

Description

This PR enables test_eigen for MSVC 2019 & 2022 C++20.

With the combination of

• re-enabling the age-old MSVC workaround even for MSVC C++20 (533e5ad)
• adding a warning suppression to eigen.h (cbb29c0)

test_eigen runs successfully with MSVC 2019 & 2022 C++20.

See also: #3722 (comment) (the entire discussion thread there)

Suggested changelog entry:

Testing eigen.h is now also enabled for MSVC 2019 & 2022 in C++20 mode.

[rwgk]

Before 533e5ad all tests passed (warnings ignored). After the commit this error is generated (copied from MSVC 2022 C++20 log):

D:\a\pybind11\pybind11\include\pybind11\cast.h(1014,1): error C2752: 'pybind11::detail::type_caster<Eigen::Ref<Eigen::Matrix<double,-1,-1,0,-1,-1>,0,Eigen::OuterStride<-1>>,void>': more than one partial specialization matches the template argument list [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]