gh-115931: Fix SyntaxWarnings in test_unparse (#115935)

Author: sobolevn

Lib/test/test_unparse.py

@@ -650,9 +650,18 @@ def test_multiquote_joined_string(self):
         self.check_ast_roundtrip("""f'''""\"''\\'{""\"\\n\\"'''""\" '''\\n'''}''' """)
 
     def test_backslash_in_format_spec(self):
-        self.check_ast_roundtrip("""f"{x:\\ }" """)
+        import re
+        msg = re.escape("invalid escape sequence '\\ '")
+        with self.assertWarnsRegex(SyntaxWarning, msg):
+            self.check_ast_roundtrip("""f"{x:\\ }" """)
+        self.check_ast_roundtrip("""f"{x:\\n}" """)
+
         self.check_ast_roundtrip("""f"{x:\\\\ }" """)
-        self.check_ast_roundtrip("""f"{x:\\\\\\ }" """)
+
+        with self.assertWarnsRegex(SyntaxWarning, msg):
+            self.check_ast_roundtrip("""f"{x:\\\\\\ }" """)
+        self.check_ast_roundtrip("""f"{x:\\\\\\n}" """)
+
         self.check_ast_roundtrip("""f"{x:\\\\\\\\ }" """)
 
     def test_quote_in_format_spec(self):