[pybind11] Let CMake download pybind11

unittest/python/pybind11/CMakeLists.txt

-find_package(pybind11 REQUIRED)
+if(CMAKE_VERSION VERSION_GREATER 3.11)
+  include(FetchContent)
+  FetchContent_Declare(pybind11
+    GIT_REPOSITORY https://github.com/pybind/pybind11
+    GIT_TAG v2.8.0)
+  FetchContent_GetProperties(pybind11)
+  if(NOT pybind11_POPULATED)
+    FetchContent_Populate(pybind11)
+    add_subdirectory(${pybind11_SOURCE_DIR} ${pybind11_BINARY_DIR})
 
-pybind11_add_module(cpp2pybind11 cpp2pybind11.cpp)
-target_link_libraries(cpp2pybind11 PRIVATE pinocchio_pywrap)
-  
-if(CMAKE_CXX_STANDARD LESS 14)
-  message(STATUS "CXX_STANDARD for cpp2pybind11 set changed from ${CMAKE_CXX_STANDARD} to 14")
-  set_target_properties(cpp2pybind11 PROPERTIES CXX_STANDARD 14)
+    pybind11_add_module(cpp2pybind11 cpp2pybind11.cpp)
+    target_link_libraries(cpp2pybind11 PRIVATE pinocchio_pywrap)
+
+    if(CMAKE_CXX_STANDARD LESS 14)
+      message(STATUS "CXX_STANDARD for cpp2pybind11 set changed from ${CMAKE_CXX_STANDARD} to 14")
+      set_target_properties(cpp2pybind11 PROPERTIES CXX_STANDARD 14)
+    endif()
+  endif()
 endif()