Compiling a melodic project in noetic: catkin_make can't find Python.h

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Hi, I'm taking over a project that was developed by a fellow student for his diploma thesis. I've received the project files but I can't get the project to compile. It's not a vast project, with only three packages and only a few dependencies. But it was developed under ROS melodic and I'm using noetic. I've checked all dependencies and there shouldn't be any issue. The only issue is a C++ library matplotlibcpp.h used to plot data. It requires the header file "Python.h" and that's the stumbling block. When I try to compile the project using catkin_make, I get the following error:

In file included from /home/jan/roboweld_ws/src/roboweld/planner/src/planner/graph_plotter.cpp:6: /home/jan/roboweld_ws/src/roboweld/planner/include/planner/matplotlibcpp.h:5:10: fatal error: Python.h: No such file or directory

I have asked another colleague, who worked on the project and she said, she was able to compile it under melodic. I know melodic uses Python 2 and noetic Python 3, but the library states it's compatible with both. I've checked /usr/include/python3.8 and the header file is there. VS Code also recognizes the header file. I'm led to believe that the issue lies with the CMakeLists.txt file. I tried to modify it, explicitly including Python directories, but that didn't help either. This is the original CMakeLists:

cmake_minimum_required(VERSION 3.1.3)
project(planner)

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_EXTENSIONS OFF)

find_package(catkin REQUIRED COMPONENTS
        rospy
        roscpp
        std_msgs
        interactive_markers
        moveit_core
        moveit_ros_planning
        moveit_ros_planning_interface
        moveit_ros_perception
        rviz_visual_tools
        moveit_visual_tools
        pluginlib
        pcl_ros
        pcl_conversions
        rosbag
        tf2_ros
        tf2_eigen
        tf2_geometry_msgs
        geometric_shapes
        )

find_package(ompl)
find_package(Ceres REQUIRED)
# find_package(sciplot REQUIRED)

find_package(Eigen3 REQUIRED)
find_package(Boost REQUIRED system filesystem date_time thread)


set(THIS_PACKAGE_INCLUDE_DIRS
        doc/interactivity/include
        )

catkin_package(
        LIBRARIES
        INCLUDE_DIRS
        CATKIN_DEPENDS
        moveit_core
        moveit_visual_tools
        moveit_ros_planning_interface
        interactive_markers
        tf2_geometry_msgs
        DEPENDS
        EIGEN3
        geometric_shapes
        ompl
)

###########
## Build ##
###########
include_directories(
        include
        ${catkin_INCLUDE_DIRS}
)

## Declare a C++ library
add_library(${PROJECT_NAME}_utils
        src/${PROJECT_NAME}/utils.cpp
        )
        target_link_libraries(${PROJECT_NAME}_utils
        ${catkin_LIBRARIES}
        )

add_library(${PROJECT_NAME}_OmplPlanner
        src/${PROJECT_NAME}/OmplPlanner.cpp
        src/${PROJECT_NAME}/utils.cpp
        )

target_link_libraries(${PROJECT_NAME}_OmplPlanner
        ${catkin_LIBRARIES}
        Ceres::ceres
        # sciplot::sciplot
        )

        add_library(${PROJECT_NAME}_graph_plotter
        src/${PROJECT_NAME}/graph_plotter.cpp
        )
target_link_libraries(${PROJECT_NAME}_graph_plotter
        ${catkin_LIBRARIES}
        )  

add_library(${PROJECT_NAME}_CeresOpt
        src/${PROJECT_NAME}/CeresOpt.cpp
        src/${PROJECT_NAME}/utils.cpp
        )
target_link_libraries(${PROJECT_NAME}_CeresOpt
        ${catkin_LIBRARIES}
        Ceres::ceres
        )


include_directories(${THIS_PACKAGE_INCLUDE_DIRS})
include_directories(SYSTEM
${catkin_INCLUDE_DIRS}
        ${Boost_INCLUDE_DIR}
        ${EIGEN3_INCLUDE_DIRS}
        ${OMPL_INCLUDE_DIRS}
        )




install(DIRECTORY launch DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION})


add_executable(approach_outside_lower_weld src/approach_outside_lower_weld.cpp)
target_link_libraries(approach_outside_lower_weld
        ${PROJECT_NAME}_utils
        ${PROJECT_NAME}_OmplPlanner
        ${PROJECT_NAME}_graph_plotter
        ${catkin_LIBRARIES}
        ${Boost_LIBRARIES}
        ${OMPL_LIBRARIES}
        )
install(TARGETS approach_outside_lower_weld DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION})




add_executable(approach_rib_weld src/approach_rib_weld.cpp)
target_link_libraries(approach_rib_weld
        ${PROJECT_NAME}_utils
        ${PROJECT_NAME}_OmplPlanner
        ${PROJECT_NAME}_graph_plotter
        ${PROJECT_NAME}_CeresOpt
        ${catkin_LIBRARIES}
        ${Boost_LIBRARIES}
        ${OMPL_LIBRARIES}
        )
install(TARGETS approach_rib_weld DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION})


add_executable(rib_weld src/rib_weld.cpp)
target_link_libraries(rib_weld
        ${PROJECT_NAME}_utils
        ${PROJECT_NAME}_OmplPlanner
        ${PROJECT_NAME}_graph_plotter
        ${PROJECT_NAME}_CeresOpt
        ${catkin_LIBRARIES}
        ${Boost_LIBRARIES}
        ${OMPL_LIBRARIES}
        )
install(TARGETS rib_weld DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION})

add_executable(outside_lower_weld src/outside_lower_weld.cpp)
target_link_libraries(outside_lower_weld
        ${PROJECT_NAME}_utils
        ${PROJECT_NAME}_OmplPlanner
        ${PROJECT_NAME}_graph_plotter
        ${PROJECT_NAME}_CeresOpt
        ${catkin_LIBRARIES}
        ${Boost_LIBRARIES}
        ${OMPL_LIBRARIES}
        )
install(TARGETS outside_lower_weld DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION})

and this is the one modified by me:

cmake_minimum_required(VERSION 3.1.3)
project(planner)

set(CMAKE_CXX_STANDARD 14 ...