Remove astunparse for newer pyython

[matejsp]

astunparse is not maintaned library and is still introducing six dependency in our projects.
It seems that it is used only 3.9 and less.
https://github.com/bufbuild/protoc-gen-validate/blob/main/python/protoc_gen_validate/validator.py#L15-L19

I propose adding astunparse to only old pythons.

Like for example:
https://github.com/pandas-dev/pandas/blob/v1.5.3/setup.cfg#L34-L39

Problematic line:
https://github.com/bufbuild/protoc-gen-validate/blob/main/python/setup.cfg#L24