Use consistent `Python::` or `Python3::` targets, in find_package, and in find_dependency

[wdconinc]

Ah, this probably should have been in the Python:: namespace since that's what is added as a dependency in the config

podio/cmake/podioConfig.cmake.in

Lines 28 to 32 in 5d82748

	if(NOT "@REQUIRE_PYTHON_VERSION@" STREQUAL "")
	find_dependency(Python @REQUIRE_PYTHON_VERSION@ COMPONENTS Interpreter Development)
	else()
	find_dependency(Python COMPONENTS Interpreter Development)
	endif()

and in the PODIO_GENERATE_DATAMODEL module:

podio/cmake/podioMacros.cmake

Line 158 in 5d82748

COMMAND ${Python_EXECUTABLE} ${podio_PYTHON_DIR}/podio_class_generator.py ${CLANG_FORMAT_ARG} ${SCHEMA_EVOLUTION_ARG} ${UPSTREAM_EDM_ARG} ${YAML_FILE} ${ARG_OUTPUT_FOLDER} ${datamodel} ${ARG_IO_BACKEND_HANDLERS} ${LANGUAGE_ARG} ${VERSION_ARG} ${OLD_DESCRIPTION_ARG}

Then again, the package that is found is Python3...

podio/cmake/podioBuild.cmake

Lines 197 to 200 in 5d82748

	if(NOT PODIO_RELAX_PYVER)
	find_package(Python3 ${REQUIRE_PYTHON_VERSION} EXACT REQUIRED COMPONENTS Development Interpreter)
	else()
	find_package(Python3 ${REQUIRE_PYTHON_VERSION} REQUIRED COMPONENTS Development Interpreter)

Originally posted by @wdconinc in #908

The mixed use of Python:: and Python3:: targets is confusing and should be harmonized. In this case, I think(!) it's ok for downstream users since find_dependency and PODIO_GENERATE_DATAMODEL are consistent...

Leaving up to @tmadlener to decide which way to go here.