CMake: Update pybind11 to 2.6.1

[knzivid]

2.6.1 is the latest version on conan. The changelog for pybind11 is at https://pybind11.readthedocs.io/en/stable/changelog.html

MISC

[knzivid]

There are some WIP commits until I get the CI green, bear with me :)

Let's resume the review once green. I can't find the "convert to draft" button

[knzivid]

Before I clean up the commits, here are the changes

• Bump the package version in Conan
  • New pybind11 in conan does not export pybind11's CMake targets. It makes its own.
  • conan's pybind11 does not bundle pybind11Config.cmake, so I can't find_package() anymore
  • conan's pybind11 does not import or link with python targets, so we manually do it
• Even in 3.6 tests, cmake can detect 3.9. We pass the python version used for setup.py down to cmake's find_package(Python3)

[knzivid]

Ready for review, commit cleanup TODO

[nedrebo]

Before I clean up the commits, here are the changes

• Bump the package version in Conan

  • New pybind11 in conan does not export pybind11's CMake targets. It makes its own.

  • conan's pybind11 does not bundle pybind11Config.cmake, so I can't find_package() anymore

  • conan's pybind11 does not import or link with python targets, so we manually do it

• Even in 3.6 tests, cmake can detect 3.9. We pass the python version used for setup.py down to cmake's find_package(Python3)

This is bigger changes then when I reviewed the previous version. I'm leaving this review to the maintainers.