test: fix warning/error on Python 2 in 14/17 mode

tests/CMakeLists.txt

@@ -151,6 +151,14 @@ function(pybind11_enable_warnings target_name)
       target_compile_options(${target_name} PRIVATE -Werror)
     endif()
   endif()
+
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
+    if(CMAKE_CXX_STANDARD STREQUAL "14")
+      target_compile_options(${target_name} PUBLIC -Wno-deprecated-register)
+    elseif(NOT CMAKE_CXX_STANDARD VERSION_LESS 17)
+      target_compile_options(${target_name} PUBLIC -Wno-register)
+    endif()
+  endif()
 endfunction()
 
 set(test_targets pybind11_tests)