[python] Re-merge Subclass Python exceptions (#7321)

Created by: Kylmakalle

Re-merging lost commit of "Subclassing python exceptions" (#7321) by @ryanrishi after "Renames python generators" (#7965) by @wing328.

https://github.com/OpenAPITools/openapi-generator/pull/7965/files#diff-54adab86675d7379d0ce4f680369a8a1161fb5427d5dc3f58e97e06d84040f52