unpin gast dependency for pip_package
Fixed some missing `=` in `!=` I will be gone for a week (due to 4th of July) but will check again when I'm back (and probably earlier on the github side) I can't really do something about the failing linter check, the line for excluding protobuf versions is pre-existing, and probably needs to be that long anyway.

I also cannot see what's failing with the Internal CI build. Other than that, everything looks green. 🥳  This worked! 🎉 

Thank you very much for pushing this through! Thanks for merging! :)
Commit c762c4501ca017994c1fa5554c3c8e47b7c80b66 noted that "I also know that gast 0.5.2 breaks one of our tests" and committed "gast versions above 0.4.0 are incompatible with some of TF's tests."

In #56244, it was suggested to raise a PR to remove the cap. In view of the above-mentioned constraints, I've excluded everything after 0.4.0 up to 0.5.2; note that 0.5.0 directly followed 0.4.0, see https://github.com/serge-sans-paille/gast/tags