Proper initialization for __version__

[DimitriPapadopoulos]

Also, AppVeyor is not used any more, is it?

I believe this is the proper way to find the version:

1. Try with importlib.metadata.version:

   The version() function is the quickest way to get a Distribution Package’s version number, as a string:

2. Fall back to version.py if running from outside a package, as a script.

[astanin]

Yes, appveyor is not used anymore.