Preliminary enhancements for Python target

Hi Ter,
as proposed I have updated to the latest antlr4 master.
I followed the manual:
 - create a branch
 - pull latest from antlr4 master
 - manually resolve merge conflicts
 - commit and push
So I was that my master branch would only contain my minor enhancements.
However, I'm still seeing numerous changes in this pull request, and I don't understand why.
Very few of these changes are mine. 
So I'm sending this pull request so you can maybe help me understand what I'm doing wrong.
Cheers