gh-92986: Fix ast.unparse when ImportFrom.level is None (GH-92992)

This doesn't happen naturally, but is allowed by the ASDL and compiler.
We don't want to change ASDL for backward compatibility reasons
(GH-57645, GH-92987)
(cherry picked from commit 200c9a8)

Co-authored-by: Shantanu <[email protected]>

Author: miss-islington

Lib/ast.py

@@ -849,7 +849,7 @@ def visit_Import(self, node):
 
     def visit_ImportFrom(self, node):
         self.fill("from ")
-        self.write("." * node.level)
+        self.write("." * (node.level or 0))
         if node.module:
             self.write(node.module)
         self.write(" import ")

Lib/test/test_unparse.py

@@ -341,6 +341,12 @@ def test_invalid_fstring_backslash(self):
     def test_invalid_yield_from(self):
         self.check_invalid(ast.YieldFrom(value=None))
 
+    def test_import_from_level_none(self):
+        tree = ast.ImportFrom(module='mod', names=[ast.alias(name='x')])
+        self.assertEqual(ast.unparse(tree), "from mod import x")
+        tree = ast.ImportFrom(module='mod', names=[ast.alias(name='x')], level=None)
+        self.assertEqual(ast.unparse(tree), "from mod import x")
+
     def test_docstrings(self):
         docstrings = (
             'this ends with double quote"',

Misc/NEWS.d/next/Library/2022-05-19-22-34-42.gh-issue-92986.e6uKxj.rst

@@ -0,0 +1 @@
+Fix :func:`ast.unparse` when ``ImportFrom.level`` is None