ast.ImportFrom.level is incorrectly marked as optional

Bug report

Bug description:

The level field on ast.ImportFrom is marked optional in the ASDL definition, but in fact the parser always sets it:

>>> ast.dump(ast.parse('from x import a'))
"Module(body=[ImportFrom(module='x', names=[alias(name='a')], level=0)])"
>>> ast.dump(ast.parse('from .x import a'))
"Module(body=[ImportFrom(module='x', names=[alias(name='a')], level=1)])"
>>> ast.dump(ast.parse('from ...............x import a'))
"Module(body=[ImportFrom(module='x', names=[alias(name='a')], level=15)])"

We should mark the field as required for consistency with the parser.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-135960