None is not a valid value for ast.ImportFrom.level

[hauntsaninja]

For example:

import ast
tree = ast.parse("from x import y")
tree.body[0].level = None
ast.unparse(tree)  # raises

This is very similar to https://bugs.python.org/issue46289 and #30467 cc @isidentical

Note the compiler does seem to allow it, but it's always an int in practice and documented as such

Discovered in typeshed.

[isidentical]

Thanks for the fix @hauntsaninja.