python/CMakeLists.txt: rework Python development files detection

python/CMakeLists.txt: rework Python development files detection

Both the FindPythonInterp and FindPythonLibs modules were removed. Use
find_package(Python3 COMPONENTS Interpreter Development) to find both
the interpreter and the library.

The old modules have been deprecated since CMake 3.12.

Resolves the following warning:

CMake Warning (dev) at python/CMakeLists.txt:11 (find_package):
  Policy CMP0148 is not set: The FindPythonInterp and FindPythonLibs modules
  are removed.  Run "cmake --help-policy CMP0148" for policy details.  Use
  the cmake_policy command to set the policy and suppress this warning.