New python import structure does not work if SIO backend is not built

[kjvbrt]

when using podio-dump compiled in nightlies stack __version__.py is not installed to install directory, when installed the circular dependency emerges:

Traceback (most recent call last):
  File "/home/jsmiesko/Work/FCC/podio/install/bin/podio-dump", line 10, in <module>
    from podio import __version__
  File "/home/jsmiesko/Work/FCC/podio/install/python/podio/__init__.py", line 25, in <module>
    from . import root_io, sio_io, reading
ImportError: cannot import name 'sio_io' from partially initialized module 'podio' (most likely due to a circular import) (/home/jsmiesko/Work/FCC/podio/install/python/podio/__init__.py)

• OS version: AlmaLinux9
• Compiler version: GCC 11.3.1
• PODIO version: d402f74=develop-z47c5e
• Reproduced by: compile latest version of podio in nightlies stack
• Input: any root file
• Output: see above
• Goal: dump a file

[tmadlener]

After some debugging, we found that:

• __version__.py is not installed when installing the python/podio directory via cmake, because of
  

  podio/python/CMakeLists.txt

  Line 22 in 0cf68a8

  REGEX .*sio.*\\.py$ EXCLUDE # All things sio related

• The uncoditional import of sio_io fails and raises a confusing error which hides the fact that ROOT wasn't able to load the dictionary in this case

[tmadlener]

Just for completeness, this was introduced with #459