Try a different python parser that is able to parse assignment expressions

[dours]

( This one https://github.com/RobEin/ANTLR4-Python-grammar-by-PEG )
This task blocks #37 and #52 , because they need all the modern python code to be parseable.

[dours]

Almost done:

• All the current tests pass locally.
• An additianal cpython test with assignment expressions works as well. (This is why a new parser is needed, the old one cannot parse these constructs)
• But CI fails due to JDK versions incompatibilities. Source is here https://github.com/polystat/py2eo/tree/tryGrammarByRobEin

[dours]

done in #94