Allow nativeparse to parse source code directly (#21260)

This is the mypy counterpart of
https://github.com/mypyc/ast_serialize/pull/54

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>