Review the generator python code

[tmadlener]

In the python generator code there are still quite a few places that were originally put in place for being compatible with python2 and python3, e.g.:

podio/python/podio_config_reader.py

Line 3 in 03f193e

from __future__ import absolute_import, unicode_literals, print_function

There are also a few compatibility imports here:

podio/python/podio_class_generator.py

Line 13 in 03f193e

# collections.abc not available for python2, so again some special care here

Additionally, the specific function for the ordered loading of the yaml file should be obsolete with python >= 3.7 (and in principle also already with python >= 3.6, though not yet guaranteed). Additionally, I think we no longer explicitly depend on the datatypes being in the same order as they appear in the yaml file:

podio/python/podio_config_reader.py

Line 301 in 03f193e

def ordered_load(stream, Loader=yaml.Loader, object_pairs_hook=OrderedDict): # pylint: disable=invalid-name

Since python2 support has been dropped some time ago (see #207 for a few more details), we could also clean up the generator code a bit.