Lost compatibility with Python2

[hegner]

It seems PODIO lost its compatibility with Python2 by introducing formatted strings in e.g.

podio/python/generator_utils.py

Line 120 in eb298b1

raise DefinitionError(f'{self.full_type} is a fixed width integer type that is not allowed')

Are we ready to give up on Python2 compatibility?

At the same time, the handling of Python2 vs Python3 is not really stable for systems, where Python 3 is explicitly called python3. And Python2 just called python, Throughout the CMake macros we just talk about python

[tmadlener]

I thought we had decided that at one of the meetings after #182 was merged, because with that we removed the last CI workflows that were still using python2. At least I stopped worrying about python2 compatibility after that, and also some of the things that are in #143 would break, e.g.
https://github.com/AIDASoft/podio/pull/143/files#diff-4dd2bca85ef3468fc6ba4f2701bc3131d54a47d333c9fa2112e85dddd5de988cR64

[andresailer]

Noted here: https://codimd.web.cern.ch/lkFym47kSdKGWqmPLCpf-w#Podio7 python3 only

[hegner]

thanks. Seems I forgot about that. I will update some of the docs

[hegner]

#209