Purpose

Declare a minimum Python version of 3.12 because FindPython3 is only available in 3.12 or after. This gives better error messages if a user is using too old a version of CMake.

Before behavior

If you install CMake 3.5, before this PR, you get this error when trying to compile sensor_msgs with colcon.

  Could not find a package configuration file provided by "Python3" with any
  of the following names:

    Python3Config.cmake
    python3-config.cmake

  Add the installation prefix of "Python3" to CMAKE_PREFIX_PATH or set
  "Python3_DIR" to a directory containing one of the above files.  If
  "Python3" provides a separate development package or SDK, be sure it has
  been installed.

New behavior

CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  CMake 3.12 or higher is required.  You are running version 3.5.0

How to prevent things like this in the future?

The CMake maintainers recommend having CI compile at the cmake_minimum_required declared in a package.
Thus, all the message packages should be compiled with 3.5. This prevents accidentally adding new CMake features and forgetting to bump cmake_minimum_required.

As far as I can tell, the build farm is using the default CMake version of each Tier 1 platform. I think it would be a lot of customization to get this right. For now, manual tests like this one can catch it.

References

• https://cmake.org/cmake/help/latest/module/FindPython3.html
• Relates to BUG: ament_cmake_core doesn't work in CMake 3.5 anymore but doesn't declare a minimum cmake version to use it #513
• Set hints to find the python version we actually want. #508 sets it to a higher version in rolling, but that can't be backported to iron or humble because of REP-2000's CMake restrictions.