Implement PrintAST for Python

[aeisenberg]

This issue is about adding support to the CodeQL VS Code extension for viewing ASTs of Python files. See the parent issue github/vscode-codeql#492.

[RasmusWL]

Hi @aeisenberg, as you might know, we're currently busy reworking large parts of the Python QL libraries. So we're not going to prioritize implementing the PrintAST functionality mentioned in github/vscode-codeql#505 before we're done with the rewrite.

[erik-krogh]

#4461 has been merged.