[3.13] gh-129958: Properly disallow newlines in format specs in single-quoted f-strings (GH-130063) (GH-132692)

(cherry picked from commit 2f8b08da47)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Lib/test/test_fstring.py

@@ -1784,6 +1784,31 @@ def test_gh129093(self):
         self.assertEqual(f'{f'{1!=2=}'=}', "f'{1!=2=}'='1!=2=True'")
         self.assertEqual(f'{f'{1 != 2=}'=}', "f'{1 != 2=}'='1 != 2=True'")
 
+    def test_newlines_in_format_specifiers(self):
+        cases = [
+            """f'{1:d\n}'""",
+            """f'__{
+                1:d
+            }__'""",
+            '''f"{value:.
+               {'2f'}}"''',
+            '''f"{value:
+               {'.2f'}f}"''',
+            '''f"{value:
+                #{'x'}}"''',
+        ]
+        self.assertAllRaise(SyntaxError, "f-string: newlines are not allowed in format specifiers", cases)
+
+        valid_cases = [
+            """f'''__{
+                1:d
+            }__'''""",
+            """f'''{1:d\n}'''""",
+        ]
+
+        for case in valid_cases:
+            compile(case, "<string>", "exec")
+
 
 if __name__ == '__main__':
     unittest.main()