Bump twine version to >= 6

[benoit-cty]

Technical changes

The previous build of Core refuses to upload, the error was:

 Uploading distributions to https://upload.pypi.org/legacy/
ERROR    InvalidDistribution: Metadata is missing required fields: Name,        
         Version.                                                               
         Make sure the distribution includes the files where those fields are   
         specified, and is using a supported Metadata-Version: 1.0, 1.1, 1.2,   
         2.0, 2.1, 2.2, 2.3.  

In the METADATA file inside the dist archive we read:
Metadata-Version: 2.4

The solution proposed here is to bump the twine version to support Metadata-Version 2.4.

I also add a check of the dist files before trying to upload them.