Set hints to find the python version we actually want. (#451)

The comment in the commit explains the reasoning behind it.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.20)
 
 project(rcutils)
 
@@ -16,6 +16,21 @@ include(CheckLibraryExists)
 find_package(ament_cmake_python REQUIRED)
 find_package(ament_cmake_ros REQUIRED)
 
+# By default, without the settings below, find_package(Python3) will attempt
+# to find the newest python version it can, and additionally will find the
+# most specific version.  For instance, on a system that has
+# /usr/bin/python3.10, /usr/bin/python3.11, and /usr/bin/python3, it will find
+# /usr/bin/python3.11, even if /usr/bin/python3 points to /usr/bin/python3.10.
+# The behavior we want is to prefer the "system" installed version unless the
+# user specifically tells us othewise through the Python3_EXECUTABLE hint.
+# Setting CMP0094 to NEW means that the search will stop after the first
+# python version is found.  Setting Python3_FIND_UNVERSIONED_NAMES means that
+# the search will prefer /usr/bin/python3 over /usr/bin/python3.11.  And that
+# latter functionality is only available in CMake 3.20 or later, so we need
+# at least that version.
+cmake_policy(SET CMP0094 NEW)
+set(Python3_FIND_UNVERSIONED_NAMES FIRST)
+
 find_package(Python3 REQUIRED COMPONENTS Interpreter)
 
 ament_python_install_package(${PROJECT_NAME})